Rebuild ui-extension automatically when sources changed

It would be nice if the ui-extension build provided some file watching functionality, so it is rebuild automatically whenever a source file changed. The same goes for running the unit tests: whenever a source file or test file changed, re-run all the tests automatically.

This can be achieved with (for example) the File Watcher plugin in IntelliJ, but it would be nice if the build provided some CLI option out of the box.