mint-arena: Change demo extension, use "$demos" to get a list of demos

Engine changed demo extension, so this is needed to get demo list now.
mint-arena: CGame API: Add CG_GETDEMOFILEINFO

Allows checking if demo is valid and get info to display in UI.
mint-arena: Use trap_GetDemoFileInfo
Merge remote-tracking branch 'mint-arena/master'

Conflicts:
	code/ui/ui_atoms.c