Recommend window border size "None"

Summary:
Adds a recommendation of border size "None" for the new KWin mechanism, which
allows window decoration plugins to recommend a size for KWin's auto border
size mode.

We want our default session with Breeze to have no window borders in order to
reduce visual clutter, make it look more in line with current design trends
and have cross platform apps look more native, in particular Kirigami apps.

This is a solution for T8707.

Test Plan:
Manually with patches to KWin. More testers desirable, needs a fully refreshed
build of Breeze, otherwise the new property is not part of the compiled shared
library for some reason.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: pedrogomes1698, hein, januz, ngraham, davidedmundson, graesslin, abetts, mart, plasma-devel

Tags: #plasma

Maniphest Tasks: T8707

Differential Revision: https://phabricator.kde.org/D13481