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