switch to gtkrevealer for props pane

Author: jcupitt

org.libvips.vipsdisp.gschema.xml

@@ -31,14 +31,6 @@
       </description>
     </key>
 
-    <key type="i" name="properties-position">
-      <default>500</default>
-      <summary>Properties pane divider position</summary>
-      <description>
-        Position of the properties pane divider.
-      </description>
-    </key>
-
     <key type="d" name="scale">
       <default>1.0</default>
       <summary>Scale</summary>