ui: add a Disconnect button on connecting and downloading screen

[illwieckz]

Add a Disconnect button on connecting and downloading screen.

Also intercept Escape key to disconnect.

Mutualize a bit the code.

[illwieckz]

According to @perturbed comment, I can't intercept Escape in map loading screen unfortunately:

26 22:43 <illwieckz> do someone knows how it is coded in Unvanquished that the map loading screen has no user interaction?
26 22:43 <perturbed> illwieckz, it's 1 frame
26 22:44 <illwieckz> 1 frame when compiling GLSL
26 22:44 <illwieckz> but not when loading assets
26 22:44 <illwieckz> and making a progress bar
26 22:44 <perturbed> no it's all 1 frame
26 22:44 <illwieckz> what
26 22:44 <illwieckz> ok
26 22:44 <illwieckz> so, no way to intercept escape?
26 22:44 <illwieckz> I was not expecting to intercept escape when compiling GLSL
26 22:45 <illwieckz> but I was hoping to do so while loading assets
26 22:45 <perturbed> no way
26 22:45 <illwieckz> :'(