[Window/Desktop Switchers] Use PlasmaComponents.Label instead of Text

The former uses native text rendering which respects anti-aliasing and hinting settings.
It also automatically follows theme colors.
Unfortunately it breaks the font.bold binding, so using font.weight here instead.

BUG: 372218
FIXED-IN: 5.8.4

CHANGELOG: Font rendering in window/desktop switchers now follows anti-aliasing and hinting settings

Differential Revision: https://phabricator.kde.org/D3330