Allow client extensions/scripts to select splash screen

[wipe2238]

Currently splash screens are always picked randomly; would be nice to have control over it, for special encounters, quest-only maps, etc.

void filename_splash( string& filename, uint16 mapPid );

• filename is picked randomly (current behavior) and stored
• script call
• use filename if it's valid (exists in PATH_ART_SPLASH, is valid format, etc); otherwise, use stored value