* big commit, mainly cleanups:
- renamed gtk frontend into "gnome", which is more accurate
- create "src/modest-platform.h" and platform-specific
src/maemo/modest-platform.c and src/gtk/platform.c
- get rid of all the #if-#else-stuff in src/, use the
modest-platform instead
- implemented modest_platform_get_file_icon_name, to get
the icon name for some file or mime-type
- move the menubar_to_menu to modest_maemo_utils_menubar_to_menu
pmo-trunk-r875
- [D] src/gtk/pixmaps/qgn_list_messagin_hdr_low_unread.png