Remove obsolete argument from Grid::countPoints()
Rename Grid::countPoints() to initGrid() and remove cruft.
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.
Reduce insanity coefficient in toUnderlyingRef()