Fixed display of navigation tree.

Nodes now have the correct icon and no + sign.
Fixed minor javascript problems.
Fixed some styles and paddings.
Changed default value of BaseAttribute::setRequired to 'true'.

This makes $attribute->setRequired() shorter and makes more sense.
Added return value to NodeModel::addAttribute to be able to chain method
calls.
Introduced first, yet very simple tests.

...much more to come!