Last fixes and additions

[GeraldKimmersdorfer]

Heyho this pr fixes a bunch of things and adds the possibility to switch to fullscreen on native build by pressing F11.

Should be the last push from my side in terms of the fancy map project.

[adam-ce]

The F11 toggle looks a lot like spaghetti code to me:

• Rendering thread notifier is pulled in to do something it has no concern with
• TerrainRenderItem too, in the future we could use for rendering a small preview of map changes. having the fullscreen code would be one more thing we would have to clean out before that works.
• the same change can be done in main_loader.qml in 13 lines (not touching so many different files).

I propose to revert that commit, and I'll merge my F11 version separately.

[adam-ce]

the rest of the changes are super cool :)