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 new architecture.md file contains a birds-eye overview of the top-level architecture
using box-drawing characters, but does not go into detail yet.
