Popover is not controllable

[stomita]

Currently the hidden property is used for representing default value, and the visibility is totally uncontrollable from outside. To be controllable from outside, change hidden property as controlled one and introduce defaultHidden property to tell the default visibility.

There are public component methods like hidden() or mouseEntered() which are not documented, and essentially not used (in any part of the rest of the code).

IMO making method public in React component is not in good manner to access the internal state - it should be totally be stateless by default.

[costagolub]

#231