Code review comment for lp://staging/~albaguirre/mir/dialogs_and_tooltips

Firstly, spec questions:
*) Huh. Why do the docs allow the client to specify the location of a dialog‽ That's surely an error.
*) Relatedly, the spec is inconsistent on whether or not dialogs can have dialog children.

Again, we need to specify input behaviour of tooltips.

And maybe something about the input behaviour of modal dialogs, but that's less complicated.

I lean towards having two spec creation functions; _spec_for_dialog_surface and _spec_for_modal_dialog_surface, *particularly* if we really do need to allow client-specified positioning of modal dialogs. _spec_for_dialog_surface has half the parameters of _spec_for_modal_dialog_surface.

Relatedly, I wonder if we should drop the “_surface” from mir_connection_create_spec_for_*_surface? They're getting a bit long.