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.