GridView - added row-level populating with example

In our project we often run into use-case of drawing calendars and timesheets which require some kind of row-level headers. This patch adds support for populating GridView on a row level (with example).

I changed the iterator().next() to get(id) - seems neater than to count on repeater being the first item in Item. In version 6 we tried to add our headers in newRowItem but due to the aforementioned assumption were failing with ClassCastException.