Improved OSD behavior

Don't ask the user for a screen configuration on login, instead, apply a sane default.
When connecting a new screen, apply a default configuration so the user gets immediate
feedback "something's happening" and then prompt for a configuration via the OSD in case
the chosen default doesn't fit.

BUG: 398816
FIXED-IN: 5.17.0

Differential Revision: https://phabricator.kde.org/D22803