Fix/restore button inheritability

[werrolf]

The purpose of the Mapbender Button is to toggle Elements on and off. Doing this properly can be a complex task.

Some Elements that have nothing at all to do with controlling other Elements are seen to inherit from Button in both the PHP and JavaScript worlds. Most commonly so they blend into the toolbar visually without trying to reinvent the markup for that, and to get the base functionality of a toggling highlight.

Recent changes to the Button as a control device for other Elements have posed compatibility issues for those "I just want to look like a Button" non-controlling Elements.

To resolve this conflict of goals, the Button JavaScript widget has been split, offering a plain, just-for-looks button under the old name mapbender.mbButton, while spinning off all functionality related to Element control into a new mapbender.mbControlButton.

The decision which to initialize is based on inheritance:

• If the Button is just the Button, it will be initialized as an mbControlButton
• If the Button is a child class of the Button, it will be initialized as an mbButton
  NOTE: of course, any PHP child class of the Button can override get getWidgetName method and thus overrule this logic. We expect most existing Button children already do this anyway.