Update dependency versions

- IJ2 to 2.0.0-beta-8
- SciJava to 2.19.2
- ImgLib to 2.0.0-beta-2
Add Border Statistics command
Add Unmix Absorbances command
Adapt commands for GeneralPathOverlays

- Fix SVG reader bug affecting relative path coordinates
- Fix Region Statistics bug affecting GeneralPath ROI bounds
- Add GeneralPath functionality to ROI Lengths command
Version 0.5.0 release

0.5.0 Release Notes
~~~~~~~~~~~~~~~~~~~

Note: This version of Slide Set is a beta release.

Updates from the previous version:

- New Border Statistics command calculates average
  pixel values in regions defined by ROI borders
- New Unmix Absorbances command separates RGB images
  into two absorbance components
- Compatibility fixes for ImageJ version 2.0.0-beta-8