Internalize parts of the goffice code

We're using only a tiny fraction of that library.
Adding it directly in the gnucash source saves a large dependency chain.

Note this is a verbatim copy of these files from
https://github.com/GNOME/goffice/tree/GOFFICE_0_8_17
to preserve a clean link with it's origin.
It will only build correctly after the next commit.

configure.ac

@@ -1697,6 +1697,7 @@ AC_CONFIG_FILES(
   doc/Makefile
   doc/examples/Makefile
   lib/Makefile
+  lib/goffice/Makefile
   lib/libc/Makefile
   lib/stf/Makefile
   packaging/Makefile

lib/Makefile.am

@@ -1,5 +1,5 @@
 if GNUCASH_ENABLE_GUI
-SUBDIRS = libc stf
+SUBDIRS = libc stf goffice
 else
 SUBDIRS = libc
 endif

lib/goffice/Makefile.am

@@ -0,0 +1,17 @@
+noinst_LTLIBRARIES = libgnc-goffice.la
+
+REALSRCS = go-optionmenu.c \
+           go-charmap-sel.c \
+           go-glib-extras.c
+REALHDRS = go-optionmenu.h \
+           go-charmap-sel.h \
+           go-glib-extras.h
+
+libgnc_goffice_la_SOURCES = ${REALSRCS}
+noinst_HEADERS = ${REALHDRS}
+
+libgnc_goffice_la_LIBADD = $(GTK_LIBS)
+
+AM_CPPFLAGS = $(GTK_CFLAGS)
+
+EXTRA_DIST = $(REALSRCS) $(REALHDRS)

lib/goffice/README

@@ -0,0 +1,13 @@
+This code is extracted from goffice.
+
+In particular,
+- go-charmap-sel is a widget to allow the user to select an encoding
+- go-option-menu is a widget used by go-charmap-sel to add manu items
+  with a radio button.
+- go-glib-extras is copied because we use a few functions from it:
+  - go_ascii_strcase_equal and go_ascii_strcase_hash
+  - go_guess_encoding
+
+This code is currently used by the csv importer, but could be useful
+in several interfaces that require the user to select an encoding
+(various importers and exporters mostly).