ui.grid.selection: add public api to enableRowHeaderSelection

[scottmcdonnell]

I can toggle multiselect using the api:
$scope.gridApi.selection.setMultiSelect(!$scope.gridApi.grid.options.multiSelect);

but there is no way to toggle the RowHeaderSelection:
$scope.gridApi.selection.setRowHeaderSelection(!$scope.gridApi.grid.options.enableRowHeaderSelection);

changing the gridOptions does not work either.
$scope.gridApi.grid.options.enableRowHeaderSelection = false;

[PaulL1]

At present the rowHeader isn't dynamic. Another instance of #1819

[PaulL1]

I don't think we're planning on allowing the rowHeader for selection to come and go - you set it up front or you don't. If you're not using the rowHeader, you can turn selection on and off dynamically, as demonstrated in the selection tutorial.

[tzhang318]

Wonder if you can allow the rowHeader to be toggleable, we are struggling to find a way to make this to work.

[Bodmoor]

You can also toggle the enableRowHeaderSelection by saving the on / off value using angular localstorage.

localStorageService.set('selectionState', true/false);

now, reload the page the grid is on and load the setting:

    var selectionState = localStorageService.get('selectionState');
    if (selectionState != null || selectionState != undefined) {
        gridOptions.enableRowHeaderSelection = selectionState;
    }

the disadvantage is of course having to reload the grid, but being able to toggle this column is great. I use it combined with the save state call, inspired by: http://plnkr.co/edit/LAMZvOkpx6jsD4CWSz04?p=preview

Also, the code samples need to be interpreted to fit your method of loading the gridOptions initially.