xsettings: Use primary monitor for scaling factor

Not screen 0.

https://bugzilla.gnome.org/show_bug.cgi?id=709859
xsettings: Check native resolution for scaling factor

Check whether the panel is running at its native resolution before
applying the scaling factor. We don't want hi-dpi screens running
below native resolution to have a high scaling factor.

https://bugzilla.gnome.org/show_bug.cgi?id=709859
xsettings: Disable scaling factor on HDMI outputs

If the output is HDMI, it's likely that the output is a TV, so
don't increase the scaling factor.

https://bugzilla.gnome.org/show_bug.cgi?id=709859
xsettings: Enable hidpi scaling on 4K monitors

Even if HDMI is used.

https://bugzilla.gnome.org/show_bug.cgi?id=709859
xsettings: Use correct flags for test application

The other test app might not have the same requirements as the
full plugin test app.

https://bugzilla.gnome.org/show_bug.cgi?id=709859
xsettings: Use gnome_rr_screen_new_async()

Since gnome-settings-daemon starts before Mutter, we need to use
gnome_rr_screen_new_async() to wait for Mutter to start and provide
the screen information over D-Bus.

https://bugzilla.gnome.org/show_bug.cgi?id=709859
xsettings: Don't mix GnomeRR and GDK information

If we don't mix information from GnomeRR and GDK, then the "changed"
signal on GnomeRRScreen is all we need to properly know when the
resolution of the system is changed - mixing them presents problems
because GDK may not yet have updated it's information from X when the
"changed" is triggered by a D-Bus signal.

We still use GDK information before we have a GnomeRRScreen to minimize
the chance of switching the window scale during startup.

https://bugzilla.gnome.org/show_bug.cgi?id=709859
xsettings: Never turn on hi-dpi support for small resolutions

If the screen is less than 1200 pixels high, don't turn on hi-dpi
support - GNOME isn't going to work with that little vertical
real estate, and it's better just to be small.

https://bugzilla.gnome.org/show_bug.cgi?id=709859
xsettings: remove check for native resolution when determining hi-dpi

A non-native resolution, can still be hi-dpi - e.g., on my laptop,
gnome-settings-daemon lists a resolution of 1920x1440 on my 2560x1440
display. The other checks we have in place should be sufficient to
disable hi-dpi support if the user picks a small resolution.

https://bugzilla.gnome.org/show_bug.cgi?id=709859