kcontrol: improve categorization

kcontrol: improve categorization

* move ksplash KCM module to kcontrol sub-directory, ksplash it
  build always and there is no reason to keep it out of the
  kcontrol directory holding all major KCM modules

* merge some sub-category modules into other major categories
  almost the same way it is done in KDE5 as it makes much more
  sense and users do not have to go to in-and-out of
  sub-categories to configure the overall desktop appearance
  - application-appearance -> desktop-appearance
  - window-behaviour -> workspace-behavior
  - workspace-appearance-and-behavior -> workspace-behaviour