[Request]: Support Filtering/Search, and Exposing appstream metadata

[Quackdoc]

A low priority set of features for sure. But it would be nice to be able to see what metadata an application has, as well as sorting specific metadata values as defined in https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html

one extremely nice metadata bit for instance is https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-relations-control which tells us how we can control the application. For instance Touch optimized applications vs gamepad optimized ones.

[WatchMkr]

We're interested in adding this information, include <url/> and <project_license/>

[WatchMkr]

Licenses and URLs have been added. Leaving open for other meta tags we may want like <control/>.