Make the project multi-module, put the JAR code and the GWT code under the same branch

[x-ji]

The backend code should be something shared by all the components and deployable to a JAR. The frontend GWT should just be a separate module that uses the backend. It seems that the new GWT-maven plugin https://tbroyer.github.io/gwt-maven-plugin/ helps organize this structure to be much clearer. We can give it a try.

[verenablaschke]

How much time/effort would this (and/or #18, #44) take? Since we do have a GWT/web version that is working nicely (and we almost have a console version), I wonder if this wouldn't take a lot of effort for relatively little gain...

[x-ji]

I'm looking at it. Shouldn't be too hard since we just need to put the existing code into the right places under the new structure and change some configurations. This will be helpful if we are to make improvements on the backend code in the future (e.g. I think issues like #40 "ignore whitespace tokens" are still unsolved?) as we will only need to modify one codebase and don't need to sync the changes across the two branches all the time.

[x-ji]

Of course if this proves to be too much trouble we can always rebase the branches to pull in the newest fix, or come up with another way to package the JAR file without having to change the gwt-plugin version.