Add scroll-method property to pointer devices

My use case is to enable middle-button scroll on my keyboard with pointing
stick. The device is recognized as USB mouse.

wiki/Configuration:-Input.md

@@ -31,6 +31,7 @@ input {
         natural-scroll
         // accel-speed 0.2
         // accel-profile "flat"
+        // scroll-method "two-finger"
         // tap-button-map "left-middle-right"
         // click-method "clickfinger"
         // left-handed
@@ -40,6 +41,7 @@ input {
         // natural-scroll
         // accel-speed 0.2
         // accel-profile "flat"
+        // scroll-method "no-scroll"
         // left-handed
     }
 
@@ -47,6 +49,7 @@ input {
         // natural-scroll
         // accel-speed 0.2
         // accel-profile "flat"
+        // scroll-method "on-button-down"
     }
 
     tablet {
@@ -124,6 +127,8 @@ A few settings are common between `touchpad`, `mouse` and `trackpoint`:
 - `natural-scroll`: if set, inverts the scrolling direction.
 - `accel-speed`: pointer acceleration speed, valid values are from `-1.0` to `1.0` where the default is `0.0`.
 - `accel-profile`: can be `adaptive` (the default) or `flat` (disables pointer acceleration).
+- `scroll-method`: when to generate scroll events instead of pointer motion events, can be `no-scroll`, `two-finger`, `edge`, or `on-button-down`. 
+  The default and supported methods vary depending on the device type.
 
 Settings specific to `touchpad`s: