LocalMode column search prototype.

Thinking of creating a method in NgReactGridReactManager that is similar to getObjectPropertyByString that gets the base key/column name for arbitrarily nested data column reference. Currently data source must have unique key names throughout all levels.

Styling and sizing is piggybacked off ngGridHeaderCell. Wonder if we should decouple some of its internal methods so ngReactGridColumnSearch doesn't need to duplicate them when moved out of thead. 

Any suggestions would be most welcome.