[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/clientenvironment.cpp

@@ -22,6 +22,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "clientenvironment.h"
 #include "clientsimpleobject.h"
 #include "clientmap.h"
+#include "clientscripting.h"
 #include "mapblock_mesh.h"
 #include "event.h"
 #include "collision.h"
@@ -42,6 +43,7 @@ ClientEnvironment::ClientEnvironment(ClientMap *map, scene::ISceneManager *smgr,
 	m_smgr(smgr),
 	m_texturesource(texturesource),
 	m_client(client),
+	m_script(NULL),
 	m_irr(irr)
 {
 	char zero = 0;
@@ -243,6 +245,8 @@ void ClientEnvironment::step(float dtime)
 		}
 	}
 
+	m_script->environment_step(dtime);
+
 	/*
 		A quick draft of lava damage
 	*/