So far we had a (commented out) entry that was called `Remove <application
package>` that would open the configured application.

* Instead of looking up the service files in PackageKit, it does so on
AppStream, allowing for a faster and sync approach.
* Renames the action to `Open <application name>`.
* Uses `appstream://` URI scheme, allowing for the system to decide which
application handles it rather than an awkward configuration entry.
* Drops PackageKit-Qt opcional dependency, adds a required AppstreamQt
dependency instead.

This also means that applications coming from other sources than the
packaging system (i.e. Snappy, Flatpak or AppImage) would also be supported,
as long as it's supported by the software center.

REVIEW: 129697