collabsible tree needs to use new deep labeling

[janpolowinski]

• only simple labeling used at the moment
• problem of repositioning the complex labels
• solve this only when the complex positioning is permanently done via positioning HTML divs, otherwise this problem is obsolete
• deep labeling of connectors is neither implemented for collapsible tree - positiioning the labels must be calculated differently than for the arcs in the force driven layout. (also cf. ticket #39)