[wayland] Implement resize_x/resize_y constraint adjustment

Summary:
Since we send the initial configure event after placing xdg-popups, we
can finally implement resize_x and resize_y constraint adjustments.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27049