Full Screen support

[RossComputerGuy]

Full Screening videos in YouTube or full screening anything doesn't work.

[morgant]

This might be resolved by adding support for the _NET_WM_STATE_FULLSCREEN atom, but I haven't researched too intensively yet.

[morgant]

I'm still learning this codebase, but we might be able to:

1. Add the Atom to InternUsefulAtoms() in mlvwm/mlvwm.c
2. Use GetMapStateProp()/SetMapStateProp() from mlvwm/event.c in a new function in mlvwm/functions.c
3. Call that function from somewhere. Most other state handling is triggered by a button in handle_button_press() in mlvwm/event.c, so I think we'd add some kind of listener to state change.

[morgant]

Upon looking into this a bit further, I think we'll actually need to implement more of the Extended Window Manager Hints to be able to enable fullscreen support. OpenBSD's fvwm, which has a compatible license, doesn't seem to support the extended hints, but OpenBSD's cwm does, so that might be a good reference. I'll likely add a separate issue for supporting extended hints.