Meson introspection step stripped the MatePanelApplet prefix, so Python
applets failed to load with a missing factory_main attribute. This only
affected mate-panel built with meson, autotools worked fine.

This fix matches the symbol/identifier prefixes with autotools so the
introspection data is generated correctly.

* Add Meson support

* Fix libegg submodule configure

* generate schemas before install

This in turn emits an 'activate' signal that any applet can connect to
and perform whatever action it means to activate in that applet.

This allows users to control certain applet functionalities through
DBus, like registering new keybindings, or integrating them with other
software.

We need to check the existence of dconf-editor's GSettings schema before
using it, because the program will abort if dconf-editor is not installed
on the system when it tries to access it.

Mostly found by cppcheck.

origin commit was:
https://github.com/mate-desktop/mate-panel/commit/96c7ebc

This reverts commit 96c7ebc6dc9e8b8327db04a9570054ee78743353.

*Fix the window list menu replacing the panel context menu between the end of the window list and the next applet

Mostly found by cppcheck.