Cleanup: move toolsystem into own include

Many files using the window manager don't access the tool-system.

This avoids rebuilding many files when the tool-system changes.

source/blender/windowmanager/intern/wm_event_system.c

@@ -80,6 +80,8 @@
 #include "WM_api.h"
 #include "WM_types.h"
 #include "WM_message.h"
+#include "WM_toolsystem.h"
+
 #include "wm.h"
 #include "wm_window.h"
 #include "wm_event_system.h"