Create continuous skeleton quality estimate widget

[jmrk84]

Can be implemented as plugin or native code. Problem: Annotators need feedback during tracing, to enforce proper and frequent node placement. The widget should display:

1. Mean inter node distance for last 10 nodes; Checkbox to enable warning when mean internode distance exceeds certain value.
2. Mean inter node time intervall for last 10 nodes; This should become red / green depending on the a settable value (i.e. too slow, make it red).

[orenshatz]

I can do such plugin easily. But there is a general concern, namely we currently don't have a mechanism of notifying python code on events in knossos, I have to see whether this is possible. Such notifification would be required for both displays when placing a node. The alternative would be an asynchronous timer in the plugin (this also has to be proved first, but should work) that simply updates the display, regardless of whether a new node has been set. I can also interface the "skeleton changed" boolean that knossos holds for unsaved-changes-upon-exit (or for timing annotation, whichever) into python, so the timer routine returns immediately when nothing has changed.