fix: remove the need for slabThickness in volumeAPI for tools

[sedghi]

• Removes the need for slabThicknessEnabled
• Simplifies the API for setSlabThickness
• Fix bugs for volumeNewImageEventDispatcher
• Fix a bug for Synchronizer that triggered source viewport event when it didn't belong to the synchronizer anymore
• Fix RectangleROIStartEndThreshold tool various bugs with the new API
• Fix arrow tool to accept external api for change/set text

[Punzo]

Nice fix. One comment: you set the infrastructure in crosshair to set the slabthickness by volumeIDs, but then in the petCt example you don't use it. You should exclude the pet volume from the third row of viewports.

i.e. now the example renders like:

but it should be (i.e., no slab thickness for the pet):

[JamesAPetts]

Minor comments, I think the all-by-default or just use these isn't very clear in the API, and minor changes could fix that.

Otherwise I agree with Davide, the petCt example needs to be updated to reflect these changes.

[JamesAPetts]

Minor comment on demo, otherwise LGTM.