Clip activation visalization

[michael-burk]

A visual indicator (at best interactive) in the editor, which shows the Preparation Time and Dispose Time.

This is especially important, when compositors are nested, as the first clip in the descendent compositor needs to start later by the preperation time, or it won't be called early enough. Also relevant for #34

[michael-burk]

Seems we need another indicator for "off time". Because when compositors are nested and the prepare of a chid starts directly with activation of the parent, the off state is never reached. A better solution would probably be to inherit the off-state.