schemas: Add tablet/stylus/pad schemas

Compared to the g-s-d schemas, there's a few notable differences:

- There are no compositor actions for rings/strips, with the libinput
  driver this is rather seen as a continuous axis, so we can't tell
  much about how it's being made discrete. It is preferred that clients
  manage the related pad events.
- Tablet area is interpreted as an array of four 0..1 doubles, being
  each of those the padding on one side, relative to the full output
  size. We can no longer use values relative to the device resolution,
  because it might not be available.
- Button mapping has been split into "primary" and "secondary"
  button settings, rather than an array of button numbers. The allowed
  values are now an enum, as button numbers are troublesome (especially
  the [4-7] scrolling button range, which can't be made to work on
  wayland)

https://bugzilla.gnome.org/show_bug.cgi?id=769034