Add property about available controls

[hadess]

This would be useful for games, to know whether they can be controlled through:

• joystick (maybe with a hint as to which type is best suited)
• keyboard alone
• mouse alone
• mouse + keyboard

See https://mail.gnome.org/archives/games-list/2016-July/thread.html#00002 for the original discussion

[ximion]

Sounds good. We can't name this input_method, because this word has meaning already.
So, something like
<input_control>mouse</input_control>
or
<input_type>keyboard</input_type>
I need to think a bit more about the name and whether it makes sense to have a tag with children for this.
@hadess Knowing nothing about joysticks, what does "joystick type" mean?

In any case, I can give this a general thumbs up, but the exact details and tag names would need to be worked out.

[Kekun]

@ximion The different controllers could be:

• gamepads
• arcade sticks
• lightguns
• joysticks (for flight simulators)
• steering wheels
• dance mats
• guitars
• trackballs
• …

[ximion]

This is resolved via #265 now. Thank you for reporting this issue!