Use stock GTK+ icons in a few more places

Partially fixes bug 689418.
Use icons from the icon naming specification

Try to use freedesktop.org Icon Naming Specification names for icons in
preference to those that EasyTAG installs by default.
Fix missing prototype warnings

Additionally, make the missing prototypes warning an error in the list
of compiler warnings in configure.ac.
Use gboolean for returning TRUE and FALSE
Remove some unused variables
Fix an uninitialised variable warning
Use g_file_new_tmp() for creating temporary file

Add a requirement on GIO 2.32 in configure.ac.
Add static to many declarations

Use the Samba findstatic.pl script to find functions which should be
static, and mark them as such. Remove unused code, as the compiler gives
warnings about unused static functions.
Use g_return_if_fail() and g_return_val_if_fail()
Avoid use of AC_CYGWIN and AC_MINGW32

Use AC_CANONICAL_HOST and AS_CASE instead.
Simplify filename_to_display()

Use g_filename_to_utf8() to convert filenames from the GLib filename
encoding to UTF-8 for display to the user.
Merge from wip
Error while make because of missing static in prototype of Id3tag_Write_File_v23Tag()
Some cleanups to stop make warnings