Implements disable-while-typing in mutter.

Disable-while-typing disables the touchpad while the user is typing.

This patch introduces the necessary backend code to implement the
org.gnome.desktop.peripherals.touchpad.disable-while-typing setting of
gsettings-desktop-schemas which was implemented in commit
4c5b1c1df399d6afaaccb237e299ccd1d5d29ddd and released as part of 3.24.
This is known as dwt in libinput.

This patch has been tested on X11 and Wayland.

https://bugzilla.gnome.org/show_bug.cgi?id=764852