[AppStream] Dependencies on things in AppStream

[Matthias Klumpp]

Here is an initial draft for requires/recommends in the spec:
https://github.com/ximion/appstream/commit/5bf623d0e0e70674922afcbb55d7176d6c7d65a7

Some notes:
I deliberately didn't add the full range of stuff that we could have
here, like disk_space, cpu or firmware items. Disk space and CPU will
need a bit more discussion first on what exactly they mean and how we
should check for them, while for firmware I'll need help from Richard
to do that properly (it's "do what fwupd does" in that case, pretty
much.
Also, I didn't include regexes or globs for version comparisons yet -
if I add those, then maybe for firmware only.
The default assumption if the compare property is missing for version
comparisons is "ge". I actually wanted "eq" there, because that would
make the most sense, but in case of the "memory" item, this would be
really confusing (do we need to check for *exactly* 2GiB then?).
Therefore, instead of making an exception for the memory item, I just
made "ge" the default for all items.

Please look at what's written in that commit and see if that is
something we could agree on for now (I expect us to extend this quite
a bit in future).

Cheers,
    Matthias