effects: Add Window View effect

The Window View effect is a companion effect for the task manager. It
provides the task manager a way to ask the user to pick a window among
windows identified by window ids passed to the activate() method.

The main motivation for adding this effect is to provide a QtQuick based
alternative for the present windows effect, which is needed to kill the
latter.

This change doesn't extend the overview effect to avoid repeating
mistakes of the past, i.e. adding too many (unrelated) features to the
present windows effect.

While the overview effect provides you a way to select windows, it's not
the only thing that it has. For example, it also allows changing virtual
desktops, etc.

On the other hand, the task manager doesn't need all of that. It only
needs to ask the user to pick a window among the specified windows, nothing
more.

As is, the window view effect is simply WindowHeap with a d-bus api slapped
on top of it.