Display configuration

[Alan Griffiths]

It could be that I'm confused, or it could be that we're confused. So
I'm starting with a quick email. If need be I'll create a doc for
discussion.

The current situation is that we have multiple incompatible
representations of the display configuration:

1. MirDisplayConfiguration and the mir_display_config_... functions.

2. MirDisplayConfig and the mir_display_configuration_... functions.

3. The mir::graphics::DisplayConfiguration interface and the various
platform implementations.

It looks as though we have started to replace 1 with 2 but haven't yet
finished.

Looking at the code it seems the various platform implementations of 3
"just" populate similar data structures to provide the same functionality.

I think there is enough commonality that a single internal
implementation based on 2 could be used across server, platforms and
clients. I'd go so far as to say that DisplayConfiguration is a concept
that ought to be in libmircore (with a suitably ABI-stable API).

There would be advantages to having a single representation (DRY) and,
as servers, platforms and clients can operate in the same process, it is
easier to use,