Add changerespawncolour() script command

This command simply changes the color that the player respawns with upon
death. The respawn color also persists through custom save files.

Closes #830.

desktop_version/src/Game.h

@@ -481,6 +481,8 @@ public:
     bool disableaudiopause;
     bool disabletemporaryaudiopause;
     bool inputdelay;
+
+    int respawncolour;
 };
 
 #ifndef GAME_DEFINITION