Refactoring of balloon panel component

[oleq]

A followup of #55.

BalloonPanel should be refactored and split into two classes: BalloonPanel and PinnablePanel (names are subject to discussion :P).

PinnalblePanel would have limited styles and arrow position configurable via model, ready to reflect arrow positions from CSS.

BalloonPanel should inherit from PinnalblePanel and bring additional styles like borders, white background and things which make it an actual panel ready to accommodate forms and toolbars like in LinkBalloonPanel. BalloonPanel should also define specific position of arrows (NE, NW, SE, SW, like in LinkBalloonPanel) or even delegate it directly to LinkBalloonPanel.

Such refactoring will allow PinnalblePanel to be reused as a parent class to create, display and attach tooltips to the buttons, like this

using the same algorithm and heuristics BalloonPanel implements at this moment, without redundancy.

[oleq]

Because #62 landed in master, I think this issue no longer makes sense. ATM BPV has more configuration and heuristics available and it doesn't look like it needed any urgent refactoring.