winemac: Don't allow double-clicks in the content area to zoom the window.

On Yosemite, double-clicking a window's title bar zooms it.  (This is to
compensate for the fact that the zoom button has been replaced by a full-screen
button.)  Sometimes, double-clicking in the content area would count as double-
clicking in the title bar.

This is controlled, in part, by the -mouseDownCanMoveWindow method of the view
that was hit in the window.  The default implementation of that returns YES
for non-opaque views, as the views are in the Mac driver.  Overriding it to
return NO prevents the problem.

dlls/winemac.drv/cocoa_window.m

@@ -374,6 +374,11 @@ - (BOOL)acceptsFirstResponder
         return [[self window] contentView] == self;
     }
 
+    - (BOOL) mouseDownCanMoveWindow
+    {
+        return NO;
+    }
+
     - (void) completeText:(NSString*)text
     {
         macdrv_event* event;