Links can now created by dragging in both directions, so now from an input port to an output port, too.
The drag-n-drop handlers now use dedicated types for each direction, so that no mismatched things can be dropped on each other.
The `View` sturct was mostly a layer of indirection, and the controller benefitted by absorbing the gtk::Application
subclass parts, so now those two are merged into a new gtk::Application subclass.
The pipewire loop now runs without interruption in a second thread and communicates with
the GTK thread via a channel in each direction, instead of checking for events once a second and using callbacks.
This allows changes to appear instantly in the view, instead of having to wait.
The controller still determines the ports media type, but instead of coloring
the port itself, the media type is passed to the constructor, which then colors the port.
