Context menu for grid items, command/shift click behaviour for selecting grid items, fix crashing bug and other small improvements.

* Added a helper method to redraw an item given its index.
* Added a IBOutlet for a context menu for an item (sets IndexSet of clicked item or selection to representedObject for any NSMenuItem in the menu).
* Added standard command click and shift click behaviours for the grid multiselect
* Option to enable and disable mouse drag selection
* Fix crash where NSNotificationCenter observers on the CNGridViewItems are not removed on dealloc
* Add public method to get NSIndexSet of visible items