Factor out 'ij1-patcher'

It was high time to separate `ij-legacy` into two different projects with slightly orthogonal purposes:

1. a runtime patcher to inject well-needed extension points into ImageJ 1.x
2. the legacy service connecting ImageJ2 with ImageJ 1.x

The project implementing the former was factored out and now lives in https://github.com/imagej/ij1-patcher. The remaining `ij-legacy` implements the latter (of course depending on `ij1-patcher` to do its thing).