mouse: Switch to new peripherals settings schema

Most peripherals settings have been moved to
gsettings-desktop-schemas.

There are some semantic differences:

* pointer and touchpad speed is now a single value in the [-1..1]
  range (from "unaccelerated" to "fast"). A value of 0 is the default;

* touchpad enabled is now an enum which can be enabled, disabled or
  disabled-on-external-mouse. This patch keeps the same UI so the last
  value is the same as disabled in the UI and can't be set for now;

* disable while typing is now always enabled so the checkbox has been
  removed;

* horizontal scrolling is always enabled when two finger scroll is
  disabled. It wasn't in the UI but we no longer need to set it since
  it doesn't exist anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=743194