Remove Swing-specific widget

The code has migrated to ImageJ:
https://github.com/imagej/imagej/commit/d0e7a1eee1683e523309588c6bbb31e99743b212
Migrate to ImageJ Common component

See:
https://github.com/imagej/imagej-common
Rename package prefix to net.imagej

This is more standard with the rest of the SciJava software stack.