[kcmkwin/kwindesktop] Fix error in desktop layout indicator initial state

Summary:
kcm.osdTextOnly is the inverse of "show desktop layout indicators"

The save was inversed, the load was not.

Also port from onCheckedChanged to onToggled as that's emitted only on
user changes and not on binding re-evaluation.

BUG: 403150

Test Plan:
Toggled it
Reopened KCM
Checked kwinrc

Reviewers: #kwin, ngraham

Reviewed By: ngraham

Subscribers: ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18272