This commit turns the ZoomOptions into a template class that
subclasses GtkDialog.
This is also a follow-up from commit c82deede1 where we started
using the non-deprecated GdkMonitor API.
Added UI and code to spawn a zoom options preferences dialog to
modify various magnifier settings. Modified the 'Seeing'
section of the universal access panel to enable the 'Options...'
button.
https://bugzilla.gnome.org/show_bug.cgi?id=643086