[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:

Reviewers: KWin, VDG, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, kwin

Tags: KWin

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