This ports the application to libadwaita, enabling us to use the libadwaita stylesheet and
widgets to better implement the Gnome Human Interface Guidelines.
The graph widgets management (watching a glib receiver, adding and removing
Nodes, Ports and Links) currently done in the `Application` and `GraphView`
objects has been extracted into a new GraphManager object, which watches the
receiver instead, pushes changes directly to the widgets, and reacts to their signals.
This seperates widget logic and management logic cleanly instead of both
being mixed into the GraphView, and also reduces the code size for the
Application object.
The new widget sits in the headerbar, and allows for changing the zoom level with "+" and "-" buttons, via text entry, and a dropdown where a list of predefined levels can be clicked.
This is more reliable than assuming the link carries the id of its nodes, as there have been cases where a link was created without those
properties set.
Instead, we can just pull them from the state via the port ids of the link.
This extends the `Application` struct to keep more advanced state.
This state is then used to determine the needed information to create
or delete a link between the two connected ports.
A message to create/delete the link is then send to the pipewire thread,
which executed the request.
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.
