There should be a way to detect when the class `open` was added/removed from a drop down-toggle

[lgalfaso]

There should be a way to detect when the class open was added/removed from a dropdown-toggle. Right now, this is not possible as this does not trigger an even nor adding/removing the class is done inside a scope::$apply.
The later option, if decided that it would be the best way to move forward, needs to be done with some care as the closeMenu is sometimes called inside an $apply and sometimes not (it is called inside an $apply when changing $location)

[pkozlowski-opensource]

@lgalfaso sorry, what is the motivation for your request?

[lgalfaso]

@pkozlowski-opensource Sometimes is needed to perform some action when the dropdown is opened/closed. In our case, we have a dropdown that has an field and we would like to focus on this element when the user clicks on it

[pkozlowski-opensource]

In this case the best approach would be to extract directive's logic into its controller so other directives can plug into this logic. Would you by up to preparing a pull request?

[lgalfaso]

@pkozlowski-opensource To make this consistent with the tabs component, created a patch that adds the open and close parameters that are called when the dropdown is opened/closed