refactor: make the single-instance feature additive

[wash2]

This is a refactor of the single instance / dbus activation implementation that makes the feature purely additive. It does add a generic parameter to Core, but I'm not sure of a better way of requiring it in run_single_instance while keeping single-instance feature additive.

[wash2]

pop-os/cosmic-settings#102 has the updates for the settings app.

[mmstick]

Would it be possible to add a DbusActivation variant to app::cosmic::Message for this? Then we could add a dbus_activation method to app::Application for handling it.

[wash2]

Would it be possible to add a DbusActivation variant to app::cosmic::Message for this? Then we could add a dbus_activation method to app::Application for handling it.

Thanks, that would work well