universal-access: Allow setting custom magnification step

Currently the step for incrementing or decrementing magnification
is decided internally, it's better if we allow user to set
the magnification step factor.

We expose another setting in zoom settings called
`magnifier_step_factor_spin` Uses mag-step-factor gsetting.
Depends on gnome-settings-daemon!269 and gsettings-desktop-schemas!52 being merged first.

Related to: gnome-settings-daemon#458
8 jobs for mag-step-factor in 4 minutes and 33 seconds (queued for 3 seconds)