Getting rid of macros and dead code:

Summary:
- convert defines and macros to constexpr variables and functions
- remove experimental code which  was introduced in
  55dd369c, and unused since
  61df57e5
- remove dead DEBUG_LAYOUT code, which wouldn't compile since
  commit efb0f40b, as that deleted a
  function used there

fix: do a comparision in Q_ASSERT, not an assignment

Reviewers: #kde_games, aacid

Reviewed By: aacid

Subscribers: aacid, #kde_games

Differential Revision: https://phabricator.kde.org/D11814