feat(dropdown): Added dropdownMenu directive to use for context menus

I have added a feature to dropdown, so it can be used as a context menu. Meaning it will show up at the specified coordinates. See demo.js and demo.html for details.

It is implemented as a dropdownMenu directive. So elements marked with dropdown-menu will register with the dropdown controller, and when dropdown is toggled, be called with the mouse event that triggered the dropdown.

I have also made a change to dropdown-toggle so it stops propagation, since this could e.g. trigger an accordion to toggle if the dropdown-toggle was placed inside an accordion header.