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

This is a series of small patches to improve the looks of the gamepad KCM, and to match a bit better the older QWidgets joystick KCM. This way the transition is easier on Plasma 6, until the brand new functionalities are eventually ported.

• 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

BEFORE	AFTER	QWidgets 'joystick' KCM

Currently depending on !1868 (merged) to simplify merging