Implementing support of `stype: "custom"` in the filter toolbar.

Hello Tony

I read recently [the issue](https://github.com/tonytomov/jqGrid/issues/444). I find the implementation of support custom controls in the filter toolbar not so complex. So I implemented it today. In the pull request I send my suggestions.

[The demo](http://www.ok-soft-gmbh.com/jqGrid/CustomControl.htm) demonstrates that the changes works.

Best regards
Oleg