Use a description language to build game types and game source types

[Kekun]

Using a descriptive language to describe the game types and the game source types could help, maybe?

Example:

<game type="SegaSaturnGame">
    <source>
        <MimeTypeTrackerQuery mime-type="application/x-saturn-rom" />
    </source>
    <name>
        <UriName />
    </name>
    <runner>
        <RetroRunner module="libretro-saturn.so">
            <path>
                <CuePath />
            </path>
            <uid>
                <SegaSaturnUID />
            </uid>
        </RetroRunner>
    </runner>
</game>

With this, we wouldn't have to define SegaSaturnGame or SegaSaturnTrackerQuery in Vala.