[decorations] Let KDecoration plugins recommend a border size per default
Instead of changing the border size default for all KDecoration plugins by
switching the default from border size Normal to None introduce new
functionality, which allows a KDecoration plugin to recommend a border size in
its metadata. By default KWin listens for these recommendations and sets the
border size accordingly.
If there is no metadata recommending a border size, KWin falls back to the
current setting of Normal sized borders.
A user is able to override the recommendations from the KCM, which has been
Test Plan: Manually with adjusted metadata of Breeze.
Reviewed By: VDG, ngraham
Subscribers: hpereiradacosta, filipf, anemeth, davidedmundson, abetts, graesslin, ngraham, zzag, kwin
Maniphest Tasks: T8707
Differential Revision: https://phabricator.kde.org/D13284