snap: show refreshable snaps in updates panel

This adds support for showing refreshable snaps in the updates panel, and then allows them to be refreshed when update is pressed. This is useful if you have manually set the snapd refresh timer so that updates don't occur automatically, to then be able to review the available updates visually.

This screenshot shows two snaps apps with updates available (kanagame a GUI app, vobsub2srt a CLI app with no app icon).

cc @robert.ancell - this is what i couldn't get polkit working with :-)

Also i understand if this wouldn't be enabled by default in Ubuntu as there could be confusion with update-manager, but I feel for distros like Debian or Fedora where their other package updates are shown here it would be useful to show snap updates alongside apt/rpm/flatpak/fwupd etc updates.