Add scroll method libinput option

<libinput>
  <device>
    <scrollMethod>none|twofinger|edge</scrollMethod>
  </device>
</libinput>

Fixes: #2766

docs/labwc-config.5.scd

@@ -941,6 +941,7 @@ extending outward from the snapped edge.
     <middleEmulation></middleEmulation>
     <disableWhileTyping></disableWhileTyping>
     <clickMethod></clickMethod>
+    <scrollMethod></scrollMethod>
     <sendEventsMode></sendEventsMode>
     <calibrationMatrix></calibrationMatrix>
     <scrollFactor>1.0</scrollFactor>
@@ -1035,6 +1036,19 @@ extending outward from the snapped edge.
 
 	The default method depends on the touchpad hardware.
 
+*<libinput><device><scrollMethod>* [none|twofinger|edge]
+	Configure the method by which physical movements on a touchpad are
+	mapped to scroll events.
+
+	The scroll methods available are:
+	- *twofinger* - Scroll by two fingers being placed on the surface of the
+	  touchpad, then moving those fingers vertically or horizontally.
+	- *edge* - Scroll by moving a single finger along the right edge
+	  (vertical scroll) or bottom edge (horizontal scroll).
+	- *none* - No scroll events will be produced.
+
+	The default method depends on the touchpad hardware.
+
 *<libinput><device><sendEventsMode>* [yes|no|disabledOnExternalMouse]
 	Optionally enable or disable sending any device events.
 

docs/rc.xml.all

@@ -635,6 +635,7 @@
       <middleEmulation></middleEmulation>
       <disableWhileTyping></disableWhileTyping>
       <clickMethod></clickMethod>
+      <scrollMethod></scrollMethod>
       <sendEventsMode></sendEventsMode>
       <calibrationMatrix></calibrationMatrix>
       <scrollFactor>1.0</scrollFactor>