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/editors/physics/particle_edit.c

@@ -82,6 +82,7 @@
 #include "WM_api.h"
 #include "WM_types.h"
 #include "WM_message.h"
+#include "WM_toolsystem.h"
 
 #include "RNA_access.h"
 #include "RNA_define.h"