Area Design Dialog fix

When using Grid layouts, after you create a new child area in the grid - class SubArea - and you click the area and choose "Edit Area Design" on the pop-up menu, the classes specified in page_theme.php for the parent area should be available. Since the SubArea instance appends a separator ":" to the area name/handle, the array keys returned by getAreaThemeClasses in page_theme.php can't match up.

Fixed area design dialog so that if its a SubArea type, it'll use the parent area handle to match against the classes from page_theme.php's getAreaBlocks method