shell: Add the ability to dynamically move application window

shell: Add the ability to dynamically move application window

So far, we could start an application on a different output, but
moving them back-and-worth wasn't really supported. This handles
a few use-cases like:

- move an application's window from one output to another
- install black curtain in case we no longer have any other surfaces
  on that output;
- deactivate and switch to the previous active window in case there's
  one available
- the activation is handled by the shell client, while deactivation
  is done implicitly by the compositor to simplify the shell client

This does a bit of rewording of a function that returns true whenever
we have only a single application window on a particular output.

Bug-AGL: SPEC-4673
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I58cde58c6a2e2dade93566bfd7aff5ed697f5450