feat(dropdown): add nesting capability

[sebastianhenneberg]

Adds nesting support for dropdowns as discussed in #2421. Allows to reflect deep hierarchies using dropdowns. The implementation uses an array as a stack to keep track of opened dropdown scopes.

[RobJacobs]

@sebastianhenneberg There is a css solution for implementing this feature here

[sebastianhenneberg]

@RobJacobs I have seen the CSS solution in the issue already. You get this with pure bootstrap (without angular-ui/bootstrap). I required a solution that utilizes the dropdown directive and allows me to customize the behaviour.

[wesleycho]

I am not sure we want to support something like this due to poor UX, although if I remember correctly, Bootstrap also had some controversy related to this as well.

[wesleycho]

Closing as per #2421 (comment)