Migrate legacy component to external repository

See:
https://github.com/imagej/imagej-legacy
Remove remnants of plugins subtree
Update README to reflect new structure
Restructure ij-app as a single "imagej" module
Merge branch 'legacy-split'

This externalizes the ImageJ Legacy component to:
https://github.com/imagej/imagej-legacy

And then migrates the ImageJ application component (formerly called
ij-app) to the root of the repository as a single-module project.
Fix application assembly

We don't need the "rootdir hack" anymore since imagej.git is no longer a
multi-module project!