Use Units in ROI/MeasurementTool

This PR adapts the ROIs/MeasurementTool to make use of the unit objects.

**Not ready for review, yet**. But if you notice problems/crashes with respect to ROIs let me know, likely to be related to this PR. 

Still to do:
- Make use of #3227 's methods for conversion in UIUtilities.transformSize() and UIUtilities.transformSquareSize()
- Remove UnitType class once it's possible to get the unit's symbol directly from the unit object. 

/cc @gusferguson @pwalczysko @jburel