[kcm/kwinrules] Improve export buttons

Summary:
Small UX improvements when exporting rules

Add a `Select All` button
Disable `Save` if no rules are selected
In export mode set button text to `Cancel Export`

Test Plan: {F8276279}

Reviewers: #kwin, #vdg, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29341