mate-desktop: Cleanups
mate-menus: Cleanup and fixes
mate-doc-utils: Fix test
mate-menu-editor: Fix test
mate-panel: cleanup

Python is not used anywhere so remove useless python check,
Remove sed on convert files, already included in latest release.
Cleanup gtk3 USE handling
mate-file-manager-share: mate_gen_build_system into src_prepare
Revert "mate-settings-daemon: Fix gstreamer config opts"

This reverts commit bc573f4029ef1d6bcf2bdf6211385e13d99ee17f.