gtk3:Some MATE integragion and backporting bits from gtk4 version

[mark-herbert42]

1. Left click on battery opens mate battery statistics dialog
2. Left click on volume open mate volume control dialog with advanced device selection, output/input profiles etc.
3. Backported language widget - now it shows keyboard language and allows to change with mouse click
4. Implemented panel unhide on keyboard language change and volume mute/unmute with keyboard shotcut
5. Backported configurable panel hide-unhide duration,
6. Added notification center size config options.

[trigg]

This is a very mixed bag PR.

Hardcoding in specific apps would be a reason to not take it, before even discussing if we intend to maintain a gtk3 fork alongside gtk4.

[mark-herbert42]

Hardcoding in specific apps

Thanks, fair point. Updated to take the config option for cmmand to run on click instead of hardcoding MATE utilities.

[mark-herbert42]

if we intend to maintain a gtk3 fork alongside gtk4.

I am 100% sure that it is not intended at all as gtk3 fork seems to be unchanged for a fairly long time - enough to understand it is abandoned. So my intention is not to keep it up to date with gk4 bleeding edge development which is too hard, but keep a kind of usable gtk3 version that can be easily integrated with gtk3 based DE's like MATE and Xfce. Those DE's will not in near future migrate to gtk4 (at least MATE which develops extremely slow this days).