Display MCS plugin

[Pasi Orovuo]

Hey everyone,

A while back I discussed in irc with Jasper about the problems with the
display mcs plugin. Among requiring a "dummy default" value, we found
that the behaviour of the Revert button isn't the most expected, as it
just restored the gamma sliders to their defaults.

Based on our discussion I made an attempt[1] to address the issues.
Basically the changes are:

 1) The "Default" option that, if selected, does nothing on startup.
 2) The revert button, which now reverts the values to those selected
    prior showing the display preferences dialog (which appeared as
    the correct behaviour to me)
 3) Resolution change confirmation. This wasn't something we agreed upon
    with Jasper, but rather something I already had written[2] and
    decided to leave there. Can and will be removed if it's found
    inappropriate (it's also the reason for Gtk+ 2.6 requirement).

For me it seems to work, but I cannot reproduce the problems people are
having in a few bugzilla entries. I understood Jasper had a way of
testing them, so I decided to "publish" the patch here for him (and
everyone interested) to test.

Thanks,
-Pasi

[1] http://medusa.tutka.fi/~pasi/display_plugin_default_revert_confirm.patch
[2] http://bugzilla.xfce.org/show_bug.cgi?id=867