dispatch UI start-up actions

[Ichthyostega]

The setup of the UI-Layer can be dissected into two distinct phases

• wiring and instantiation of all participating global services
• activation of the UI event loop

This creates the generic problem that all actual start-up tasks, which really need an operative UI, somehow need to be scheduled and dispateched from phase-1 into phase-2. The task for this ticket is to build and install a facility to achieve this. Preferrably this facility should live within the UiManager, for reasons of thematic relatedness

[Ichthyostega]

After a lot of code-reading it became clear that GTK is not of much help here

• we decided to use GTK-the-Toolkit
• we do not want to buy into GTK-the-Framework
• consequently, Lumiera is not based on a Gio::Application
• but rather we call the small number of really relevant GTK startup-functions ourselves

Thus, basically Glib::Dispatcher can be ruled out, because it requires an alleady running event-Loop,
and since we don't have an Gio::Application, we also don't have a signal_activation() -- the latter being the mechanism
used by Gtk internally to show the GTK Application Window (which we don't need, we just use several Gtk::Window widgets)

[Ichthyostega]

In the end, I decided against building a generic service here, since it pretty much looks like a one-time problem.
Preferrably UI content will be pushed or pulled on demand, rather than actively coding content from within the UI-Layer.

The solution is to attach to Glib::signal_timeout() from the ctor of the InteractionDirector.
This signal offers a connect_once() operation, and is implemented by attaching a event source (closure) to the
g_main_context_default() -- which is even created on demand. Thus we're able to place something really
into the event loop, even before gtk_main() has been invoked.

[Undercover Agent]

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