Avoid nesting Controls in TextField

Summary:
Label now has an extra mousearea conflicting with TextField's mouse
area.

Given we're in the overriding font and colour anyway, we may as well use
Text directly to resolve the bug.

It also saves creating an unused Accessiblity attached object.

One extra property is copied from our Label control for complete parity.

BUG: 406851

Test Plan: Moved the mouse about in systemsettings

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20792