Introducing /snap/patches directory

This patch introduces the /snap/patches directory for packagers to place
patches used in the packaging process.  A `patches` part is included to
install these files under $SNAPCRAFT_STAGE/packages to be used by other
parts.

A Sed script is also included to patch the desktop entries' name and
icon.

Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>

snap/patches/README.markdown

@@ -0,0 +1,3 @@
+# /snap/patches
+
+Here are the patches that are required to build a snap package that functions correctly.

snap/patches/patch-desktop-entries.sed

@@ -0,0 +1,28 @@
+# This sed script patches the desktop entries of the snapped
+# application.
+#
+# Documentation:
+#
+# * GNU Sed Manual
+#   https://www.gnu.org/software/sed/manual
+#     * `sed` script overview - `sed` scripts
+#     * `sed` commands summary - `sed` scripts
+#     * The `s` Command - `sed` scripts
+#     * Overview of basic regular expression syntax - Regular
+#       Expressions: selecting text
+#     * Back-references and Subexpressions - Regular Expressions:
+#       selecting text
+
+## Append '(Snappy Edition)' to the application name to make it
+## distinguishable with the other same application using different
+## software distribution technologies
+##
+## FIXME: The appended string is not localizable, the proper way to
+##        implement this is to probably use a new X-Snappy-Name keys
+##        with localestring format to let the translators fill in
+##        additional localized string and use these values to replace
+##        the Name keys here.
+s/^\(Name\(\[.\+\]\)\?=.*\)$/\1 (Snappy Edition)/g
+
+## Fix-up application icon lookup
+s|^Icon=.*|Icon=\${SNAP}/meta/gui/icon.svg|

snap/snapcraft.yaml

@@ -24,6 +24,47 @@ apps:
     #slots:
 
 parts:
+  # Patches to fix other parts
+  patches:
+    source: snap/patches
+    source-type: local
+    plugin: dump
+
+    # DISABLED: Bug #1775582 “`organize:{ /: another-dir/ }` causes the items under host root directory to be copied in another-dir” : Bugs : Snapcraft
+    # https://forum.snapcraft.io/t/organize-another-dir-causes-the-items-under-host-root-directory-to-be-copied-in-another-dir/5806
+    #organize:
+        #/: patches/
+    override-build: |
+      set -eu
+
+      snapcraftctl build
+
+      mkdir \
+        --parents \
+        "$SNAPCRAFT_PART_INSTALL"/patches
+      mv \
+        "$SNAPCRAFT_PART_INSTALL"/*.diff \
+        "$SNAPCRAFT_PART_INSTALL"/*.patch \
+        "$SNAPCRAFT_PART_INSTALL"/*.sed \
+        "$SNAPCRAFT_PART_INSTALL"/patches \
+        || true # Empty patches folder is allowed
+    stage:
+    - patches/*
+    override-prime: 'true'
+
   my-app:
+    after:
+    - patches
+
     # See 'snapcraft plugins'
     plugin: nil
+
+    override-stage: |
+      set -eu
+
+      snapcraftctl stage
+
+      sed \
+        --file "${SNAPCRAFT_STAGE}"/patches/patch-desktop-entries.sed \
+        --in-place \
+        "${SNAPCRAFT_STAGE}"/share/applications/*.desktop