Add an uninstall button for already installed flatpaks?

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

## Describe the bug

I installed [Aqueducts from Flathub](https://flathub.org/apps/details/com.endlessnetwork.aqueducts) via its *.flatpakref file but the AppCenter is unable to detect it.
This leads me to believe that maybe it's a good idea to add an **"Uninstall"** button for right in Sideload to make things easy and intuitive for users.

### To Reproduce
Steps to reproduce the behavior:
1. Go to https://flathub.org/apps/details/com.endlessnetwork.aqueducts
2. Click on Install
3. Open the downloaded *.flatpakref file.
4. Install it.
5. Go to AppCenter and look for Aqueduct

### Expected behavior

There should be an Uninstall button in the AppCenter for AqueDuct.

## Platform Information

- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version