Fix VR session toggle not changing reliably on session start/end

The text and icon were supposed to change but didn't reliably, which was
a race condition I think. It depended on how fast the OpenXR runtime
would transition the session state.
This also makes sure the correct notifier is sent on session exit.

source/blender/makesrna/intern/rna_xr.c

@@ -38,7 +38,7 @@ static bool rna_XrSessionState_is_running(bContext *C)
 {
 #  ifdef WITH_XR_OPENXR
   const wmWindowManager *wm = CTX_wm_manager(C);
-  return WM_xr_session_is_ready(&wm->xr);
+  return WM_xr_session_exists(&wm->xr);
 #  else
   UNUSED_VARS(C);
   return false;

source/blender/windowmanager/intern/wm_operators.c

@@ -3711,6 +3711,8 @@ static void wm_xr_session_update_screen(Main *bmain, const wmXrData *xr_data)
       }
     }
   }
+
+  WM_main_add_notifier(NC_WM | ND_XR_DATA_CHANGED, NULL);
 }
 
 static void wm_xr_session_update_screen_on_exit_cb(const wmXrData *xr_data)