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.

wiki/Configuration:-Input.md

@@ -25,6 +25,7 @@ input {
     }
 
     touchpad {
+        // off
         tap
         // dwt
         // dwtp
@@ -35,9 +36,11 @@ input {
         // tap-button-map "left-middle-right"
         // click-method "clickfinger"
         // left-handed
+        // disabled-on-external-mouse
     }
 
     mouse {
+        // off
         // natural-scroll
         // accel-speed 0.2
         // accel-profile "flat"
@@ -46,6 +49,7 @@ input {
     }
 
     trackpoint {
+        // off
         // natural-scroll
         // accel-speed 0.2
         // accel-profile "flat"
@@ -53,6 +57,7 @@ input {
     }
 
     tablet {
+        // off
         map-to-output "eDP-1"
         // left-handed
     }
@@ -122,6 +127,10 @@ Most settings for the pointing devices are passed directly to libinput.
 Other Wayland compositors also use libinput, so it's likely you will find the same settings there.
 For flags like `tap`, omit them or comment them out to disable the setting.
 
+A few settings are common between input devices:
+
+- `off`: if set, no events will be sent from this device.
+
 A few settings are common between `touchpad`, `mouse` and `trackpoint`:
 
 - `natural-scroll`: if set, inverts the scrolling direction.
@@ -137,6 +146,7 @@ Settings specific to `touchpad`s:
 - `dwtp`: disable-when-trackpointing.
 - `tap-button-map`: can be `left-right-middle` or `left-middle-right`, controls which button corresponds to a two-finger tap and a three-finger tap.
 - `click-method`: can be `button-areas` or `clickfinger`, changes the [click method](https://wayland.freedesktop.org/libinput/doc/latest/clickpad-softbuttons.html).
+- `disabled-on-external-mouse`: do not send events while external pointer device is plugged in.
 
 Settings specific to `touchpad`, `mouse` and `tablet`: