Initial support for popup window handling

Summary:
So far KWin did not properly handle popup windows. That is when a popup
surface got created and a click outside the surface happened KWin did not
send out the popupDone Wayland event.

This change makes KWin aware of whether a surface is a popup and tracks
through a new PopupInputFilter whether there are popup windows. In case
there are popups the new filter waits for mouse press events and cancels
the popups if the press does not happen on any surface belonging to the
same client. To quote the relevant section of the Wayland documentation:

    The popup grab continues until the window is destroyed or a mouse
    button is pressed in any other client's window. A click in any of the
    client's surfaces is reported as normal, however, clicks in other
    clients' surfaces will be discarded and trigger the callback.

So far the support is still incomplete. Not yet implemented are:
 * support xdg_shell popup windows
 * verifying whether the popup is allowe...