Check network status with NetworkManager before doing background updates.

Check network status with NetworkManager before doing background updates.

The logic is:

- If we're on-line, check right away.

- Otherwise (offline, connecting, unknown), sleep for 20 seconds and if
  we're not offline at the end, go ahead.

This is designed to prevent two problems:

- You log in. 0launch tries to check for updates to everything in your session
  while you're still connecting. These things therefore never get updated.

- You're offline, but 0launch displays notifications about failing to update
  from time-to-time.

If you don't use NetworkManager, then the effect of this is to always delay
updates by 20 seconds, which shouldn't cause any problems.