make selectedTag accessor public again

This reverts commit cfee75b6e9.

AppKit/CPPopUpButton.j

@@ -241,7 +241,7 @@ CPPopUpButtonStatePullsDown = CPThemeState("pulls-down");
 /*!
     @ignore
 */
-- (int)_selectedTag
+- (int)selectedTag
 {
     return [[self selectedItem] tag];
 }

Tests/AppKit/CPPopUpButtonTest.j

@@ -40,7 +40,7 @@
      withKeyPath:@"weight"
          options:nil];
 
-    [self assert:20 equals:[button _selectedTag]];
+    [self assert:20 equals:[button selectedTag]];
 
     [button selectItemAtIndex:1];
     // simulate selection by the user (this fails because we don't reverse set the value)