Rename Joystick KCM to Game Controller

Summary: Most people will use it for calibrating game controllers and the icon was recently changed to a game controller.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21922