GtkStyleContext will be deprecated in gtk 4.10.
https://docs.gtk.org/gtk4/class.StyleContext.html
This preserves code blocks where additional GtkStyleContext operations
were used, such as gtk_style_context_save/restore.
Boy this was hard.
To ease the pain of porting wireless-security to GTK4, add
a new WsFileChooserButton class that mimics the behavior of
a button that triggers a filechooser, as per the migration
guide suggests.
There were lots of GtkGrids, so the diff is particularly
horrendous. Sorry.
This needs serious testing before landing.