Mir has no function to create modeless parented dialogs

Mir dialog creation functions are insufficient. We have:

mir_connection_create_spec_for_dialog: Modeless dialogs without parenthood
mir_connection_create_spec_for_modal_dialog: Modal dialogs with parenthood

There is no function to create a modeless dialog with parenthood (which is actually very common).

We probably need a single generalized dialog function that optionally takes a parent pointer and a bool for modality.