Add support for changing subpixel order under XFCE

This also fixes the case (as on XFCE) where gsettings is present but
the org.gnome.settings-daemon... schema is not. Before this commit,
calling gsettings on XFCE with the nonexistent schema was failing and
causing thinkpad-rotate to terminate early.

Note that xfconf-query works properly when thinkpad-rotate is called
manually by the user but does not seem to work when thinkpad-rotate is
called from thinkpad-rotate-hook.
Make thinkpad-rotate disable conflicting subpixel

When two displays are attached and their subpixel orders differ, the
best option is to disable subpixel rendering entirely to avoid weird
appearance on one of the displays. If only one display is present or if
their rotations match, choose the subpixel order that looks good.
Merge remote-tracking branch 'jturner314/fix-subpixel'