gs-plugin-loader: Improve handling of pending install queue

These changes contain several closely related things:
* use a GsAppList for the install queue - helps to avoid duplicated in the file
* store more than just the app ID in the file - to install chosen app variant
* install after start and make it interactive - thus there is not waited
  with the queue for the network change and the user is asked for the credentials
  if needed, without which the app installation just fails
* store changes in the install queue more frequently - like when an install
  succeeds or fails, save the changes, to not try again.

Closes #1744
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