Add "edid-file", if we have one (in the KMS case, where we can point
people to the right sysfs file), or "edid" with inline data.
These are needed by colord to build the default ICC profile for
uncalibrated displays.
https://bugzilla.gnome.org/show_bug.cgi?id=705670
Instead of keeping a forest of if backend else ..., use a subclass
and virtual functions to discriminate between XRandR and the
dummy backend (which lives in the parent class togheter with the
common code)
https://bugzilla.gnome.org/show_bug.cgi?id=705670