Magnifier: change default focus-tracking key

Where the default for focus-tracking was 'push' this has meant that
focused items would often fall out of range, because coordinates
for 'pushing' are taken from the top left hand side. Changing the
default for focus-tracking to proportional allows the focused widget
to not be pushed out of range.

Bug:https://bugzilla.gnome.org/show_bug.cgi?id=721146