Refresh button is sensitive even if offline

### Problem

The refresh button is sensitive even if you're disconnected from network, and clicking it behaves as if it's really checking for updates, which looks strange.

### Proposal

Make the refresh button insensitive when disconnected from network or/and suggest users to connect to network to check for updates.

### Prior Art (Optional)

AppCenter shows a banner for this:

Check for Updates menuitem is sensitive though: