xdg-shell: allow clients to indicate if a toplevel is a dialog to avoid tiling

Compositors that implement tilers for WMs tend to use the heuristic min_size == max_size to determine if a window should be floating (and not tiled normally). This is generally used for clients to tell the window management that a window is a dialog window.

Technically this breaks the "policy, not mechanism" motto.

There are a few solutions I can think of:

1. Should xdg-shell allow specifying a sort of "usage" for a toplevel. This kind of creeps into WM_WINDOW_ROLE territory imo.
2. Should clients use xdg_popup for dialogs? (dialogs can be moved so probably not)
3. A new xdg-shell role object for dialogs?
4. "works as intended" or declare this to be compositor policy