[RFE] Support installing Flatpak Add-ons

Many Flatpak apps from Flathub have Add-ons (extensions) available to be installed to extend their functionality. One of these apps is Steam. In GNOME Software they are displayed on the application details page and can be easily installed from there. However, MintInstall does not seem to display them at all (on latest Linux Mint 20.1). And what is even worse, when trying to search these Add-ons, no results are found either.

You can try this yourself by trying to search for "Proton". When running flatpak search proton, these results are found:

Name                                Description                                                                                  Application ID                                     Version       Branch Remotes
Proton (community build)            Latest Proton version built on the Flatpak Freedesktop SDK                                   com.valvesoftware.Steam.CompatibilityTool.Proton   5.13-4        stable flathub
Proton-GE (community build)         Latest Proton-GE version built on the Flatpak Freedesktop SDK                                ?m.valvesoftware.Steam.CompatibilityTool.Proton-GE 5.21-GE-1     stable flathub
Proton experimental (community bui? Latest Proton experimental version built on the Flatpak Freedesktop SDK                      ?.valvesoftware.Steam.CompatibilityTool.Proton-Exp 5.13-20210107 stable flathub
protontricks                        A simple wrapper that does winetricks things for Proton enabled games.                       com.valvesoftware.Steam.Utility.protontricks       1.4.3         stable flathub

Reproduce the report in MintInstall by searching for "Proton" and compare the results with flatpak search proton. Trace how Flatpak search results and application details are handled; done means Flatpak add-ons are displayed for applications and can be found through search and installed.