Issue/#470

Addresses issue #470

When there is only one item in the drawer collection, a data attribute is added to the navigation-drawer-toggle-button, that has the value of the eventCallback from the item.

DrawerView.toggledrawer() triggers the eventCallback if one exists on the navigation-drawer-toggle-button.

Added an attribute called _forceNoBackButton to the Drawer object so that DrawerView.openCustomView() isn't the only way to hide the backbutton.