Add the autovisibility functionality for the menubar.

If /sim/menubar/autovisibility/enabled is true, show the menubar when the mouse hits the upper edge of the window and hide the menubar on click outside a pui element.

mice.xml

@@ -24,6 +24,17 @@ current mode for each mouse is held in the
    <constrained type="bool">false</constrained>
    <pass-through type="bool">true</pass-through>
 
+   <button n="0">
+    <binding>
+     <condition>
+       <property>/sim/menubar/autovisibility/enabled</property>
+     </condition>
+     <command>property-assign</command>
+     <property>//sim/menubar/visibility</property>
+     <value type="bool">false</value>
+    </binding>
+   </button>
+
    <button n="2">
     <binding>
      <command>property-assign</command>

preferences.xml

@@ -505,6 +505,10 @@ Started September 2000 by David Megginson, david@megginson.com
   <menubar>
    <visibility type="bool" userarchive="y">true</visibility>
    <default include="gui/menubar.xml"/>
+   <autovisibility>
+    <enabled type="bool" userarchive="y">false</enabled>
+    <edge-size type="int">5</edge-size>
+   </autovisibility>
   </menubar>
 
   <gui>