spearmint: Add drag and drop demo playback

The demo does not have to be in home or base virual file system directories.
Does not work on MacOSX yet (needs info.plist changes and SDL newer than 2.0.3).
spearmint: Draw loading screen when loading demo
spearmint: Reload CGame when demo plays completely

When a demo is playing and player presses a key, disconnect command is run
and cgame is reloaded. However if the entire demo is played, cgame is not
reloaded. CGame doesn't handle demo to main menu transition (not everything
gets cleaned up). It seems best to only have a single case for demo ending;
so let's always reload CGame.