Mapbender element widgets registry for better initalization handling.

The new Mapbender.elementRegistry is the better way to handle widget
initialization. Register for your target widget to be ready and have
your callback fired whenever that happens or immediately when the
target is already in a ready state.
Use new elementRegistry for initialization
Enable custom autocomplete widgets in SearchRouter
Typo in ZoomBar template fixed
Merge branch '3.0' of github.com:mapbender/mapbender into 3.0

Conflicts:
	src/Mapbender/CoreBundle/Resources/public/mapbender.element.legend.js
Merge branch '3.0' of github.com:mapbender/mapbender into 3.0
Merge branch '3.0' of github.com:mapbender/mapbender into 3.0