Alow to add an extra button in KPluginSelector
This originated from kxmlgui!2 (closed) where we agreed to settle for a button in the plugin selector.
This MR "only" allows us to display the extra button, the implementation for the KRunner usage dialog will be another big project!
To give you a "proof of concept" implementation to try this out: https://invent.kde.org/plasma/plasma-desktop/-/commits/work/puginselector_extra_button_demo
