Some misc build improvements

[okaestne]

A collection of smaller improvements and fixes:

• build:
  • migrate from intltool to gettext
  • run glib-compile-schemas for non-debian builds
  • fix build warnings for plugins
  • map: check return value of clutter_init
  • postasa: replace deprecated *_GET_INSTANCE_PRIVATE usage
  • send by mail: remove unused variable
• send-by-mail:
  • fix runtime assertion error gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed when closing xviewer with the send-by-mail plugin enabled. This is caused by the removal of the action group on exit. Refer to the documentation of gtk_ui_manager_ensure_update().
  • free the action group gobject.