Broken ownership in buttonsrebind and sticky keys plugin

The buttons rebind and sticky keys plugin have a class that extends from both KWin::Plugin and KWin::InputEventFilter.

Plugin instances are owned by PluginManager. InputEventFilters are deleted by InputRedirection. This can cause double deletes of the object.

Noticed this while working on an autotest for sticky keys. The fact that this doesn't seem to blow up right now seems just coincidental