Do clamping in the model and add missing lower bound check

Summary:
Clamping the window delegate size to stay within the desktop
rect was previously done in the QML code, but only for the
higher bound. This moves clamping into the C++ model and adds
a lower bound.

BUG:390888

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10736