Make GUI files reusable in customized GeoBatch modules

[etj]

At the moment, when creating a custom GB build, the src/webapp files have to be copied manually in the new module. We need a way to automatically import the original files from geobatch web app module.

[etj]

The idea is to use maven overlays, and then split the main geobatch .war in two:

• a core .war, only containing GUI related files to be shared with customized geobatch modules
• a final .war, adding the core .war as overlay and using all the existing related dependencies

[etj]

In src/gui there are only the MVC controllers. Would be a good idea to merge this stuff into the core GUI module.

[etj]

Changes committed in this branch: https://github.com/geosolutions-it/geobatch/tree/issue197-web-core-refactor
Please test.

[etj]

Some new work on https://github.com/geosolutions-it/geobatch/tree/issue197-web-core-refactor-2, rebased on current master.

Now the produced WAR should be fine, the webapp can be run with

   mvn jetty:run-exploded

and can be run using Start.java both from NB and Eclipse.

Since running Start.java was loading the required libs both from WEB-INF/lib and from the mvn classpath, creating so dup issues, a new ClassLoader has been implemented that skips libs from WEB-INF.

Please note that while NB loads dependant libs from .m2/repository, Eclipse may use the files in the target/ directory of depending projects if they are in the current working set. The implemented solution should work in both cases.