Remove QSaveFile in favor of plain old file saving

Summary: Rationale: for many use cases that e.g. have acls, complex other extended attributes, static links e.g. the rename() doesnt do the trick it should other ways would be start to add workarounds to all cases, which is hard, e.g. if that is something shared via SMB...

Test Plan: make && make test

Reviewers: dhaumann, dfaure

Reviewed By: dhaumann, dfaure

Subscribers: dfaure, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D14890