[RFE] Add support for retrieving Group property from Package

[Conan-Kudo]

As of hawkey 0.6.3-1, it has been possible to access the "Group" tag information of a package through the Package class. This became available in Fedora with hawkey-0.6.3-2.

In Mageia, we are working on developing applications around dnfdaemon, though unlike Fedora, we actually use RPM Groups. Like SUSE, we use them for visually categorizing packages when being presented through our installer (DrakInstall) or our GUI package management frontend (currently rpmdrake).

We're working on a new tool, dnfdragora, which we're planning on rebasing from the raw DNF API to dnfdaemon. In order for us to fully move over from the raw DNF API to dnfdaemon, we need the ability to retrieve that property so that we can sort packages into their component sections.

While it is not currently mentioned in the DNF API documentation, it is accessible the same way the rest of the properties are (through the Package class), as evidenced by the commit that added it to hawkey.

I'd greatly appreciate it if it was possible to retrieve that information along with the rest of the package properties for populating the GUI via the dnfdaemon API.

[anaselli]

Retrieving in some way a list of groups the package belongs to would be good as well.