Comment 3 for bug 1513139

I can complete this. Looking for feedback.

I would like to propose possible solutions:

1. appcenter-daemon periodically runs 'apt update'
  - there would be a lag on updates

2. appcenter runs 'apt update' on launch
  - this would make appcenter unresponsive while waiting for the updates.
  - appcenter would require user authentication to check for updates

3. appcenter optionally runs 'apt update' on launch
  - a dialog could be created "should I check for new updates"

4. appcenter provides a check for updates button