D7897: Move X11 specific Workspace initialization code into dedicated method

[Martin Flöser]

graesslin added a dependency: D7856: Make xstacking order dirty handling work 
without X11.

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D7897

To: graesslin, #kwin, #plasma
Cc: plasma-devel, kwin, bwowk, ZrenBot, progwolff, lesliezhai, ali-mohamed, 
hardening, jensreuterberg, abetts, sebas, apol, mart