Add capability to check whether network connection is metered

For now this only works with NetworkManager.  The related settings are
greyed out on systems not using NetworkManager.

Some details of the implementation:
- Implement settings in the settings menu to enable/disable feed
  updates, episode downloads and/ or image downloads on metered
  connections.  If the option(s) is disabled, an overlay dialog is shown
  with options to "not allow", "allow once", or "allow always".
- If the network is down, no attempt is made to download images and the
  fallback image will be used until the network is up again.
  This also solves an issue where the application hangs when the network
  is down and feed images have not been cached yet.
- Next to this, part of the cachedImage implementation in Entry and Feed
  has been refactored to re-use code as part of the image() method in
  Fetcher.
- In case something unexpected happens, an error will be logged.