check for unused dependencies and remove them

### Problem

Hello,

I (sometimes) think it could be great to check for unused dependencies ( cf. `flatpak uninstall --unused` ) and remove them automatically in the appcenter (i.e old version of runtimes/platforms which are no longer necessary)

### Proposal

Incorporate flatpak uninstall --unused within appcenter

### Prior Art (Optional)

_No response_