Add an option to force fullscreen

[Dareka826]

It would be useful for cage to have a commandline switch that forces its clients to be fullscreen.

This would prevent black bars in applications that don't take the full screen space when maximized.

At least that's what I think is happening in #254.

This is a problem especially for terminal emulators.

[mjt0k]

FWIW, in multi-monitor mode (-m extend, the default), fullscreen means something.. interesting, - it literally is full screen, consisting of multiple monitors. I'm trying to find a way to be able to use a single monitor for a terminal emulator, not full screen.

[joggee-fr]

@Dareka826
For the black bars issue, I need to create a new PR from #278 to rebase and propose separately the commit that should fix the issue.
However, it may be nice to add an option in Cage to fullscreen applications instead of maximizing them. In the same topic, implementing wlr foreign toplevel management could also be an interesting option to do so, see issue #99.

@mjt0k
This multi-monitor mode is by design, see display configuration in wiki.
With wlr foreign toplevel management protocol support, we may use set_fullscreen to select only one output.

Patches are welcome.