Surfaces with type mir_surface_type_dialog need appropriate decorations

[Robert Ancell]

Applications that open dialogs will create surfaces of type mir_surface_type_dialog. These surfaces need to be appropriately decorated / handled by the launcher so they are usable.

A first step might be to centre them over the application window and fade it out. Clicking outside the dialog would close it. The default policy of fullscreening surfaces will probably be a bad experience for dialogs that tend to lay out better when small.

[Robert Ancell (robert-ancell)]

This feature is required to support simple legacy applications (i.e. single window applications with dialogs e.g. a save file dialog).