Improve output enablement recovery and config write

Summary:
Remember or calculate position and mode size after an output has been enabled
again. Additionally on disable do write back the last working output config
to the config file but not to the global one.

Test Plan: Tested on Wayland and X11.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24038