Add a way to choose the preferred repository

[loxoron218]

Is your feature request related to a problem? Please describe.
When using Droid-ify, I always like to install the app Breezy Weather. When installing that app though, Droid-ify always pulls the latest version with the higher number, ignoring the "FREENET" extension.

Describe the solution you'd like
Would it be possible to choose the preferred repository per app, as the regular F-Droid client seems to do?

Describe alternatives you've considered
I've considered turning off some repositories, so the app can only be downloaded from the special "FREENET" repository that the Breezy Weather developer offers. That would mean though that I'd have to turn off the IzzyOnDroid repository and lose access to too many apps I also use.

Additional context
This is how apparently the official F-Droid client solves this problem:

[DJCrashdummy]

this would be nice to have, but even more important is the general prioritizing of whole repos: #713.