Cleanup: remove unused includes in Compositor

And move unneeded includes in frequently used headers
to source files.

Slightly reduces compile time.

source/blender/compositor/intern/COM_ExecutionModel.cc

@@ -17,6 +17,7 @@
  */
 
 #include "COM_ExecutionModel.h"
+#include "COM_CompositorContext.h"
 
 namespace blender::compositor {