Disable the title bar separator by default

Summary:
Since the removal of the heuristics this seems to be very much disliked by many.
As there is now a configuration option, disabling this by default seems like
the best option.

This diff's purpose is mainly to raise this topic for discussion.

Reviewers: #vdg, #plasma, ngraham, abetts, romangg

Reviewed By: #vdg, #plasma, ngraham, abetts, romangg

Subscribers: romangg, mart, abetts, rizzitello, mmustac, broulik, anthonyfieroni, januz, rikmills, anemeth, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10429