Add support to prioritize repositories

**Problem**
Due to my unstable Internet connection, some repositories are available, but not all of them are as fast at the same time.
Then prioritizing them would be a good idea to somehow avoid such problems, since:

- When multiple repositories have the same version of an app, the one with a higher priority is shown and to download from first.
- Otherwise, new versions go first.

**Solution**
Add a manual sorting function to the repository list. Users can simply drag them up and down to prioritize them.

**Alternative**
Add an option to select the *major repository*, and always fetch APKs from this repo first (not so good but simpler).