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 (closed) being merged first.
Related to: gnome-settings-daemon#458