Upgrade to Core 20, add wayland watcher

Upgrades the snap to core20 base, where we can pull in WPE libraries (libwpe, wpebackend-fdo, wpewebkit) from the package archive instead of building them from source in wpe-webkit-libs. Also bumps cog to 0.6.0 (from source since focal repos still have 0.4.0) and adds a inotifywait watcher to restart the browser when the wayland socket goes away because mir-kiosk is refreshed.