app: more gimp_directory() -> gimp_directory_file()
app: change GimpImageMapTool::get_settings_ui()'s "default_folder" to GFile
app: use g_file_query_file_type() to check for the GIMP directory
app: more cleanup in the action history code, mostly general consistency
app: use gimp_directory_file() in gimp_plug_in_manager_get_pluginrc()
app: port about_dialog_load_logo() to GIO
app: use GIO to find the locally installed user manual
app: more filename -> GFile in gimp-fonts.c
app: port menus_clear() to GIO