Showing only top level of grouping headers in case of usage groupCollapse:true option

The current implementation of `groupCollapse:true` option display *all* groping headers and hide only the data like one can see on the picture

<img src="http://i.stack.imgur.com/saNJr.png"/>

instead of showing only groping header of the top level group:

<img src="http://i.stack.imgur.com/Pxrfb.png"/>

[The answer](http://stackoverflow.com/a/19207377/315935) discuss the problem in details.