* Move GTK compatibility macros into a separate header
* Fix Cairo compatibility checks
* Move Glib-namespaced definitions to GtkCompat header
* Remove unused stringify definitions since we're using fmtlib
* Use `Gio::List{Model,Store}` for torrents
Switch from `Gtk::{TreeModel,ListStore}` in preparation for cell
renderers deprecation in GTK 4.10. That will require switching to the
new view classes (`Gtk::{Column,List}View`) which only work with `Gio`
models. Implement an adapter to support GTK+ 3 where the old view class
(`Gtk::TreeView`) only works with `Gtk` models; it is effective enough
but requires a signal connection per item to notify on row changes.
Refactor filtering and sorting (which now happen over the new model) to
use compatible `Gtk::Filter` and `Gtk::Sorter` classes. Although these
classes are only present in GTK 4, the abstraction is suitable for GTK+
3 as well so make our subclasses work for both versions.
Since items (of `Torrent` class) of the new model provide only a very
limited (by design) layer of compatibility with GTK+ 3 way of doing
things, refactor selection handling to do it the new way. Move selection
helpers into `MainWindow` to abstract them away since new view classes
handle it differently.
* Improve session load performance based on profiling results
