launch a web app in a system browser tab, instead of a webview

Change to launching web apps in a new tab of the default browser, instead of in a webview.

- dial back logging of job start and stop
- fix an NPE parsing pubspecs
- bind the embedded server to any port, not a fixed port number
- launch into a system browser tab (`window.open()`)
- fix an issue serving workspace files with spaces in the path (fixes #1392)

@keertip