Add directional popups

[Grimpper]

Hello, here are a few changes I've been using in my fork of popper that I thing would be of interest.

New features added:

• Addition of a transient interface to interactively select in which direction the user wants to display the popups.
• Popper will try to fit popups in free slots of the selected side of the window respecting `max-side-slots'.
• Allow the user to configure the display actions used by popper-display-function' via the user configurable variable popper-display-action'.
• Allow the run time configuration of `popper-display-function', this is required for the dynamic transient interface but could be used for other functionality such as modifying the way popper handles the display of popups according to the context.

Directional popups screenshot: