Improve first play usability

[benkeks]

EndgameKane brought up some usability issues in https://www.youtube.com/watch?v=DPRK2DCu4-k .

Immediate fixes:

• It should be easier to switch to full screen on first start.
• The spawn point selection info should be clearer.
• Maybe inform player if they enter the commander mode...
• Add a button to quit the game in overview mode. (Back in the day, all shooter games were quit with Esc—apparently, this is not obvious anymore..)

Also think about some ingame tips that can be deactivated... (Like info on first usages of stuff, first death...)

[benkeks]

Addressed the first two points by adding a toggle button for fullscreen/windowed on welcome screen 6a09208 and redesigning the spawn point selection promt 3757c01.

[benkeks]

Added leave-game button in 62739d6.