Split GUI into its own jar

[hlxid]

Closes #102 by modifying the package task to also create a jar for the GUI. Note that the gui task still has to be executed to actually build the GUI, package will just create a jar with the built GUI files.

Also adds a custom GUIServlet to serve the gui over the http server because jetty wasn't able to do it if the GUI isn't included in the same jar (at least to my testing).

I've also replaced the installGuiDeps and buildGui with a more general executeNpmCommand to increase code reuse, because the two previous methods were almost the same.

[sebinside]

Building the new gui jar works fine, including the deployment.

But when i try to start the framework from inside IntelliJ IDEA, i got the GUI couldn't be found on the classpath! Has the GUI been built? error message, although the jar is in the target folder. Am I missing something?

[hlxid]

Try to reload the sbt configuration inside IntelliJ (sbt window > reimport) to let IntelliJ discover the new jar.
This is only required for the first time, similar to the required IntelliJ reload after the api has been fetched for the first time.

[sebinside]

Wow, I did not think that the solution is that easy.