[desktopchangeosd] Sync desktop switch OSD font size to Plasma OSD font size

Summary:
Plasma's OSDs for volume/screens etc use PlasmaExtras.Heading, this
should to.

Also correct centering text to constrain the text element and align the
text within that bounding box. Previous code could have overflown in the
event of a really really long desktop name if we're resizing to the
grid.

BUG: 356600

Test Plan:
Switched desktop with OSD on
Tried with and without "show desktop layout indicators"

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16420