REQ: Allow filtering by source

[Quackdoc]

It would be nice to allow filtering by sources, With both packagekit + flatpak installed the user is likely to have 2 sources, 3 sources with "system" and "user" flathub setup.

However it is still possible to have multiple flatpak sources, and if opendesktop is added, there can be even more sources, As such it would be helpful to be able to show only specific sources.