kcms/gamecontroller: Improve the looks to match the older joystick KCM

A series of small changes to improve the looks of the gamepad KCM, 
and to match a bit better the older QWidgets `joystick` KCM.

* Improve the display of pressed buttons: Instead of a plain 0/1,
show a "PRESSED" string when a button is pressed, or a placeholder hyphen when is not.
* Fix table headers: Use `Qt::DisplayRole` ('display') instead of custom roles, 
so the ViewHeaders can access the header data.
* Make the 'Buttons' and 'Axis' tables have the same width