Summary:
Some people play Sudoku on the train, others play
Gredelbuil (grep, remove, build), i.e. manually cleaning up
includes and forward declarations which are not needed

Done by primitive grepping for Q/K* class names and
* removing any includes whose class does not have instances
  by a quick look in implementations in the same file
* using forward declarations instead of includes for classes
  whe possible, unless a template class (because the latter
  is quite verbose as forward declarations, the include
  nicer to read)
* removing any forward declarations which are not used in the
  same file
(one day a bot/script will steal my game and beat me, o tempores)

WRT effects on dependent code, at least kdevelop will need one or
two adaptions due to changed include visibility. That is done as
part of a similar clean-up patch, which will be either directly
pushed or also passed through review, as you prefer.

Reviewers: #kdevelop, apol

Reviewed By: #kdevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5071