Draft: Port To GTK4
Starts the initial port of Podcasts to GTK4.
Some TODOs for before merging:
- Figure out using MenuModel for show menu
Figure out custom menu w/ radio buttons for rate menu
(may be blocked on GNOME/gtk#3260)
- Set tnum on rate menu
- Figure out setting the right icon sizes for player dialog
Some TODOs to consider for after:
- Port ListBoxes and FlowBoxes to use models, perhaps use GridView instead of FlowBox for shows
- Port cover images to GtkPicture instead of GtkImage
You may also run into a critical when closing a FileChooser. I think this is a GTK issue.