[decorations] Let KDecoration plugins recommend a border size per default

Summary:
This is an alternative solution to T8707 and in comparision to D13276 a less
drastic change to KWin's default behavior.

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
extended accordingly.

Test Plan: Manually with adjusted metadata of Breeze.

Reviewers: #kwin, #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: hpereiradacosta, filipf, anemeth, davidedmundson, abetts, graesslin, ngraham, zzag, kwin

Tags: #kwin

Maniphest Tasks: T8707

Differential Revision: https://phabricator.kde.org/D13284