Add some MATE integration

I am currently running MATE with wayfire, and MATE definitely far away from wayland readyness. Panel runs- but many applets are not working in wayland session, and so do many panel features. Panel can not be free-floating, can not hide or autohide. And mate-panel in wayland session has noticeable CPU usage - not huge, less that 1% but compared to wf-shell it is much worse.

So wf panel looks to be a better choice here - but thinking of implementing some mate-integration that may be usefull.

1. Volume control applet - good thing but very basic. Would be nice to add right-click menu allowing to execute mate-volume-control or pavucontrol application which allows extended control over sound system.
2. Battery applet - same stoy, adding menu with possibility to call mate-power-statistics app, which show battery details and history, and battery status of bt mouse.
3. Menu - right click menu to run mozo - mate menu editor. Also making separate folder for Mate apps not to mix them with "other de's"

Those changes will not require much programming if they are hardcoded , little bit more if they are configurable (f.e. put options "volume-control-app" where you can put executble - pavucontrol or mate-volume-control, menu editor, power-statistics-app - so it not mandatory to be MATE programs but can be any favorite 3d party tool)

4. clock applet - mate clock applet so far is the best I ever seen, because this thing allows to set up several locations with easy switch between them (if you frequently travel between 2 places you can simply make your clock show time wgere you currently is instead of switching your system timezone) , and shows the weather. This one is more complex and requre other DE libs. So most likely additional applet mate-clock is needed for this.

The issue names no files, tests, or entry points. Start by narrowing the requested MATE integration to one applet and locating the corresponding wf-shell component; confirm the intended external application behavior and define which specific applet interaction and acceptance checks would constitute done.