Fixing usage of `formatDisplayField` and collapsing/expending behavior of multi-level groping

Hello Tony,

during writing [the answer](http://stackoverflow.com/a/19207377/315935) on the stackoverflow I fond some small bug in `formatDisplayField` (see the demo from [the answer](http://stackoverflow.com/a/16152739/315935)). Additionally I suggest to change the behavior of `groupCollapse:true` to that only top-level grouping headers are displayed. At the end the suggested changes modify `groupingToggle` so that only **the next** grouping sub-level will be displayed on expanding instead of displaying all sub-levels with all data currently.

[The demo](http://www.ok-soft-gmbh.com/jqGrid/ExpandingStateOfSubGroups.htm) demonstrates suggested changes. You can compare the same demo with the current jqGrid code [here](http://www.ok-soft-gmbh.com/jqGrid/ExpandingStateOfSubGroups0.htm).

Best regards
Oleg