Support interactive window selection through the Platform

Summary:
A new virtual method is added to Platform:
startInteractiveWindowSelection

The interactive window selection enters a mode where the user can select
a window through the pointer or keyboard device. The cursor is turned
into a crosshair cursor, unless another cursor name is provided (e.g.
pirate for kill window).

Once a window is selected the provided callback method is invoked with
the selected Toplevel as argument. In case the user cancelled the
selection a nullptr argument is passed in.

Currently it's only implemented by the X11 standalone platform using the
logic from KillWindow. Just instead of killing the window the callback
is invoked.

KillWindow loses the X11 implementation and interacts with the new
functionality in Platform by providing a lambda function for the
killing.

Test Plan: Killing of X11 windows is still possible

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://p...