improve Video.SliceParts by adding all the possible parameters

The documentation for Video.SliceParts just speaks about two parameters:

• duration
• xpath

I have checked the source code of this operation (available at https://github.com/nuxeo/nuxeo-platform-video/blob/master/nuxeo-platform-video-core/src/main/java/org/nuxeo/ecm/platform/video/tools/VideoSlicer.java).
This operation includes some extra parameters. Its
documentation is not up to date in explorer:

• startAt
• duration
• encode
• segmentsPath
• videoSlicer
• videoSlicerByCopy
• videoSlicerBySegments
• videoSlicerStartAt