schemas: Add a way to turn off "PC-style" right-click

On Macs, there's no physical touchpad buttons, and right clicks are
performed through 2-finger clicks.

On Lenovos (and many other PCs), specific areas of the device are set
aside to act as buttons.

The default configuration for those is to leave it as is the default for
that specific hardware, but allows to remove button areas on PC laptops.

See equivalent, old-style work-around:
https://gist.github.com/hadess/6779414

headers/gdesktop-enums.h

@@ -176,6 +176,14 @@ typedef enum
   G_DESKTOP_TOUCHPAD_HANDEDNESS_MOUSE
 } GDesktopTouchpadHandedness;
 
+typedef enum
+{
+  G_DESKTOP_TOUCHPAD_CLICK_METHOD_DEFAULT,
+  G_DESKTOP_TOUCHPAD_CLICK_METHOD_NONE,
+  G_DESKTOP_TOUCHPAD_CLICK_METHOD_AREAS,
+  G_DESKTOP_TOUCHPAD_CLICK_METHOD_FINGERS
+} GDesktopTouchpadClickMethod;
+
 typedef enum
 {
   G_DESKTOP_DEVICE_SEND_EVENTS_ENABLED,

schemas/org.gnome.desktop.peripherals.gschema.xml.in

@@ -39,6 +39,11 @@
       <summary>Natural scrolling</summary>
       <description>Set this to TRUE to enable natural (reverse) scrolling for touchpads.</description>
     </key>
+    <key name="click-method" enum="org.gnome.desktop.GDesktopTouchpadClickMethod">
+      <default>'default'</default>
+      <summary>Click method</summary>
+      <description>How to generate software-emulated buttons, either disabled ('none'), through specific areas ('areas'), number of fingers ('fingers') or left as hardware default ('default').</description>
+    </key>
   </schema>
   <schema gettext-domain="@GETTEXT_PACKAGE@" id="org.gnome.desktop.peripherals.keyboard" path="/org/gnome/desktop/peripherals/keyboard/">
     <key name="repeat" type="b">