[CSM] sound_play & sound_stop support + client_lua_api doc (#5096)

* squashed: CSM: Implement register_globalstep
  * Re-use fatal error mechanism from server to disconnect client on CSM error
  * Little client functions cleanups

* squashed: CSM: add core.after function
  * core.after is shared code between client & server
  * ModApiUtil get_us_time feature enabled for client

src/guiEngine.h

@@ -139,6 +139,7 @@ class MenuMusicFetcher: public OnDemandSoundFetcher
 class GUIEngine {
 	/** grant ModApiMainMenu access to private members */
 	friend class ModApiMainMenu;
+	friend class ModApiSound;
 
 public:
 	/**