Assorted cleanups, detailed list below.

- Avoid filename duplication in source code subdirectories:
  instead of BTGame/Strings.h and BTMEditor/Strings.h use
  BTGame/GameStrings.h and BTMEditor/MapEditorStrings.h etc.
- Move setting-strings into separate header.
- Start using forward declarations of classes more extensively in header
  files instead of including more header files to cut down recompilation time
  after small changes in headers.