D19001: katetextbuffer: refactor TextBuffer::save() to better separate code paths

[Luca Beltrame]

lbeltrame added reviewers: dhaumann, cullmann.

REPOSITORY
  R39 KTextEditor

REVISION DETAIL
  https://phabricator.kde.org/D19001

To: mgerstner, dhaumann, cullmann
Cc: kwrite-devel, kde-frameworks-devel, gennad, michaelh, ngraham, bruns, 
demsking, cullmann, sars, dhaumann