Add "off" and "disabled-on-external-mouse" properties to input devices

This is called "events <mode>" in Sway, but we decided to use more abstracted
form for consistency with the other config items. "disabled-on-external-mouse"
is added only to touchpads, but there might be other devices that support this
option.

I think "off" also applies to keyboards, but I'm not going to add the one
because we don't have libinput machinery for the keyboard config, and it's
unlikely that user wants to disable _all_ keyboards. OTOH, pointer devices can
be disabled per type. Perhaps, this should be revisited after implementing #371.

resources/default-config.kdl

@@ -21,6 +21,7 @@ input {
     // Next sections include libinput settings.
     // Omitting settings disables them, or leaves them at their default values.
     touchpad {
+        // off
         tap
         // dwt
         // dwtp
@@ -28,9 +29,11 @@ input {
         // accel-speed 0.2
         // accel-profile "flat"
         // scroll-method "two-finger"
+        // disabled-on-external-mouse
     }
 
     mouse {
+        // off
         // natural-scroll
         // accel-speed 0.2
         // accel-profile "flat"