Change the control visibility by using css class

Instead of changing the button opacity in the control, only add/remove
a css class (ol-hidden). This let the developer easily change the
behavior by updating the css.
Merge pull request #2717 from fredj/rotate-css

Change the control visibility by using css class
Consistent circle transform

Depending on compilation flags, applying a transform used to either:
- work;
- fail throwing an abstract method not implemented message;
- fail silently.

Now it should consistently work, like the other geometries.

Adding a polygon factory method creating an approximation of a circle on
a plane would be useful for users wanting the circle to be deformed.
It would be similar to the `circular` function which creates an
approximation of a circle on a sphere.
Merge pull request #2696 from gberaudo/consistent_circle_transform

Consistent circle transform.
Snapshot