Fix tablet finder button registration

Summary:
Replace nativeEvent with mousePressEvent and get rid of XCB code (button
numbers are still X11 specific, though). Qt's XCB backend should have
support for up to 31 extra button now.

Keep "Cancel" button enabled all the time

Button mapping also seems to cover stylus buttons, not just express
keys, so edit the label accordingly

Test Plan: kde_wacom_tabletfinder shouldn't skip button mapping part anymore

Reviewers: fvogt

Differential Revision: https://phabricator.kde.org/D10674