ENH: Fix PDF Segmenter to work with Prometheus

First, the PDF segmenter retreived the EditUtil module by calling the
slicer.modules.EditorWidget. It is fine as long as the editor isn't
integrated in another module. In that case, we don't want the integrated
editor to reference to the module in slicer.
Since Effect already have a accessor on EditUtil, we can simply refer to it
(the PDF segmenter is an effect).

Also, the PDF segmenter was using the command slicer.cli.run without
passing a CLI. This would create a new CLI everytime it was called.
Instead, we can create our own PDF segmenter.
A detail is that the normal PDF segmenter CLI node needs to be created
before the custom editor cli node. Otherwise the latter will be the one
 selected when opening the PDF segmenter CLI module.

Change-Id: I9b578d18c59a246ff6cc18ca73daa5a93c16791b
ENH: Use parameter node in PDF Segmenter

This allows outside module to specify inputs, outputs and parameters to the
PDF segmenter.

Basically, the parameter node is used to store all the necessary parameters
to run the PDF segmenter. Whenever the effect is shown, the default values
are checked. If any value is set on the parameter node, the value is set to
the GUI as well.
The parameter node is observed by the effect allowing the GUI to update
itself dynamically from it. On the other hand, modifying the GUI propagates
to the parameter node.
This apply() function can be factorizated in the PDF segmenter logic,
making it easier for outside module to use it if they need to.

Finally, the main input node selector was removed to be more editor-like.
Instead of getting the main volume from the GUI, it's obtained through the
editor's function (i.e. it's the current background volume).

Change-Id: I6c1be3db807d14c9afc1ddb79a1c4e210bf9f511