Supporting USB video devices

1012-09-13: postponing a remaining item, but desrt can deal with it if he's time as it's not binding to a specific milestone.

== Rationale ==
There are two broad types of USB display hardware on the market: adapters that provide only a display, and adapters that provide a display plus a USB hub to connect a keyboard and mouse.

For the first type, users (generally) wish to activate an external display in their existing session.
For the second type, users (generally) wish to get an entirely separate session on each device.

== Goal ==
To pick up any easy tasks - xorg.conf.d snippets, configuration changes, upstream cherry-picks - to make these two scenarios easier for users. It is not expected that either of these scenarios will work out of the box, although if logind is adopted for other reasons the second scenario should work without additional user configuration.

== Release Notes ==
To be filled in once we know what is actually feasible.

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Actions:
Questions from IRC:
(09:46:26 PM) xranby_ac100: i have come across some usb mimo monitors with integrated touchpad. the device is basically a usb hub that connects to a usb hid input device for the touch and a display link screen.. the hard part are how to associate the touchscreen orientation with the usb screen
(09:46:49 PM) xranby_ac100: can we support these kind of usb screens and make the touch work?

RAOF, 2012-06-04: There's not going to be cherry-pickable stuff from hybrid-graphics, but it's landing in Xserver 1.13 and there's a good chance that we'll pick that up.

Work items:
[desrt] Look at how Fedora 17 does this - apparently they do it perfectly (for the multiseat case): POSTPONED
[raof] Check out airlied's work around this - is there cherry-pickable stuff from the hybrid graphics that'll make this easy?: DONE