Snap package: cleanup, partly fix extension manager

Now the extension manager starts and has all needed dependencies. Some more fixes will probably be required until it is really usable.

Note: In the experiments leading to this build, there were problems with python crashing due to segfault. This was caused by incompatible versions of GI / PyGObject between the snap gnome extension and the Ubuntu packages. The solution was to remove all packages from the "stage-packages" and "build-packages" list that are already provided by the gnome extension.

Main issue: #5414