A favorites system to get notified of updates without installing

## Prerequisites
- [✓] I have searched open and closed issues for duplicates.

There is an issue about wishlisting but it makes no mention of update notifications, only of comparing prices.

## Feature
**Is your feature request related to a problem? Please describe.**

Sometimes I find an app that I really like, but it frankly isn't ready for prime time. It happens actually quite often because apps on AppCenter (and open source programs in general) are often pet projects or at least developed by people who also have day jobs. It would be nice to have some way to get notified when an app updates (maybe even just major versions) without having to keep it installed.

**Describe the solution you'd like**

A favorite or wishlisting system. When an app is in a user's favorites, the user will receive a notification when it updates, even if the user does not have the app installed. This would also enable a user to save a list apps to more easily remember to donate later.

**Existing work**

Many shopping websites have wishlists but I am not aware of any app stores that notify you of updates when an app is on your wishlist.

**Describe alternatives you've considered**

An alternative might be to have a "recently updated" section with prominently featured changelogs, so the user could (albeit taking on a more active role) keep track of changes themselves. This might not scale well to a large app store.