Script for finding unneeded headers

Script for finding unneeded headers

The script does the following:

  All .cpp and .h files in the current directory and subdirectories
  are checked to see which include statements could be omitted without
  causing a build error.

Many of these omissions would not be desired. For example, currently
if you don't include Undo.h in Undo.cpp, there is no error because
Undo.h is included in Cursor.h which is included in Undo.cpp. But
clearly we do want to include Undo.h in Undo.cpp.

See #6305.