slidingpopups: Support animating show/hide of input method panel

This adds support for animating showing/hiding of the input method panel to the sliding popup effect, if the input panel is of type "Toplevel".

This is mainly intended to animate showing the virtual keyboard and has been primarily tested with Maliit. It replaces the client-side animation that Maliit would do, instead doing the animation on the KWin side which provides a significantly smoother experience.