layout: Add a property to tell whether we're running on a smartphone

We're going to adjust a few things in the shell UI to allow running gnome-shell on smartphones. To do this, it will be necessary to detect whether we're running on a phone, so add a property to layoutManager that tells us.

Note that the heuristics to detect phones here is obviously quite subjective and a bit tricky, right now it's simply checking whether the primary monitor size is smaller than 500x1000, which works fairly well and excludes smaller tablets. Given that the whole web seems to rely only on checking the screen size in pixels, this seems to be not as bad as one might think.