Add a ZOOM_TO_FOCUS_[ON_]CONTENT zoom strategy

[sailingKieler]

With #24 KLighD offers zoom to fit strategy that doesn't fit the current diagram clip node's bounds into the available canvas area, but the bounding box of its displayed contents.
This make a difference for hand-drawn layouts with large container nodes and little content.

Such a strategy variant would be beneficial for the zoom to focus strategy, too.
Thus, this issue demands for a ZOOM_TO_FOCUS_[ON_]CONTENT entry in ZoomStyle.java and the required implementations.