icons: revamp and cleanup

Make a stronger icon for layer moving, and update the layers window icon
to match.

New brush-mods-{active,inactive} icons. The old ones were annoying me by
not being pixel-aligned.

Distinguish more clearly between icon sources and resources.

Update icon exporter script to also save scalable versions (with a
little manual help in Inkscape). This requires a new template for all
icon sources.

Replace the weak frame edit mode icon with a nicer ormolu picture frame
icon derived and SVG-ified from the Fatcow Hosting set: CC-By 3.0 (US).
Themeable.

Clean out some old pixmaps/ icons we no longer use.

Update LICENSE for new frame icon, and remove no-longer relevant clause.

LICENSE

@@ -1,3 +1,6 @@
+Program Code
+------------
+
 * The MyPaint application and any part of MyPaint not explicitly licensed
   otherwise, is licensed under the GNU GENERAL PUBLIC LICENSE version 2
   or later -- see the 'COPYING' file in this directory for details.
@@ -6,21 +9,63 @@
   and is licensed under the ISC license. Read 'brushlib/COPYING' for
   details. The MyPaint brush collection is not part of that library.
 
+
+Non-themeable program artwork and icons
+---------------------------------------
+
+These images are required for MyPaint to operate correctly, and are not easily
+replaceable at runtime. They should be considered "linked" in the GPL sense.
+
 * The icons eye_*.png and lock_*.png in the pixmaps directory are licensed
   under the GNU GENERAL PUBLIC LICENSE VERSION 2, and were extracted from
   tango_icons.svg in the Inkscape distribution. They are created by and 
   copyright 2009 the Inkscape Developers, with contributions by "jEsuSdA 8)
   www.jesusda.com" -- see the 'COPYING' file in this directory for details.
 
-* The icons tool_*.png are public domain, and are either taken from the
-  the Tango Desktop Project (http://tango.freedesktop.org/) or have been
-  drawn specially for MyPaint using the Tango palette.
-
 * The pixel-only icons in glade/icons/ have been adapted from those in the
   Glade distribution itself. They are licensed under the "GNU General Public
   License as published by the Free Software Foundation; version 2 dated June,
   1991".
 
+* Cursor images are taken from the DMZ Cursors 0.2 distribution.
+  These are Copyright (c) 2006 Jakub Steiner <jimmac@novell.com> and
+  Copyright (c) 2006 Novell, Inc. They are distributed under the terms of
+  the Expat License (described as the "MIT License") -- see the file
+  'COPYING.cursors' in this directory for details.
+
+Other icons in the pixmaps/ directory are distributed under the same terms as
+MyPaint itself.
+
+
+Themeable icons
+---------------
+
+Images of these sizes and names are required for MyPaint to operate correctly,
+but are easily replaceable by the user at runtime. They are installed under the
+default "hicolor" Freedesktop.org icon theme, but may be replaced by icons from
+any user theme. They should thus not be considered "linked" in the GPL sense.
+
+* Original themeable icons whose sources are in svg/ have been released into
+  the public domain. We use the public domain Tango palette.
+
+* Some PNG-only icons found under desktop/ are taken from the Tango
+  Desktop Project (http://tango.freedesktop.org/), and are Public Domain.
+
+* Some of the themeable icons in svg/ are derived from Fatcow Hosting's "3000"
+  set through tracing and redrawing into SVG. These derivative works are 
+  licensed under a Creative Commons Attribution 3.0 (US) License,
+  http://creativecommons.org/licenses/by/3.0/us/ . When attributing, please
+  link to this page on fatcow.com: http://www.fatcow.com/free-icons
+
+SVG sources embed license information on a per-icon basis as Dublin Core
+metadata. Themeable icons with licenses which would be incompatible with the
+GNU Public License under closer linking, such as CC-By 3.0, should eventually
+be replaced with ones which are.
+
+
+Artists' Resources
+------------------
+
 * The brushes and their icons from Set #2 are by David Revoy, and are released
   as cc0/public domain: https://creativecommons.org/publicdomain/zero/1.0/
 
@@ -37,11 +82,17 @@
   distributed under the terms of the GNU GENERAL PUBLIC LICENSE Version 2 --
   see the 'COPYING' file in this directory for details.
 
-* Cursor images are taken from the DMZ Cursors 0.2 distribution.
-  These are Copyright (c) 2006 Jakub Steiner <jimmac@novell.com> and
-  Copyright (c) 2006 Novell, Inc. They are distributed under the terms of
-  the Expat License (described as the "MIT License") -- see the file
-  'COPYING.cursors' in this directory for details.
-
 * The "Watercolor" palette is by Tuomas Kuosmanen (@tigert), and is released
   into the Public Domain: https://twitter.com/tigert/status/268938113575960576
+
+For resources, licenses which would restrict artists choice over licensing of
+their own work were a palette, background image, or brush to be embedded
+automatically into a file created with MyPaint MUST be avoided if any
+possibility of such automatic embedding exists with current MyPaint code.
+
+No facility exists at present for embedding palettes, but brush program code
+currently does embed. We currently require Public Domain licensing, or CC0, for
+brushes and background images as a consequence. Should MyPaint add a facility
+for embedding palette data automatically into artwork, those palettes licensed
+under the GPL shall be dropped from the distribution.
+