Allow users to choose whether they want to show apps from added repo in AppCenter

## Problem

Users can't opt out of adding the entire repo to AppCenter, so if they sideload a single app from, say, Flathub, all Flathub apps will appear in AppCenter, which some people (such as me) might not want.

## Proposal

Add an option for users to choose whether they would like to 'Show apps from this source in AppCenter'. If so, proceed as it does now. If not, add the repo anyway (so the app can be sideloaded) and hide apps from it from AppCenter

## Mock-up