gwt-maven-plugin refactored to avoid duplication

The main reason for the change is to have single common `<compileSourcesArtifact>` list that can be shared between both community and product builds. The specific Home Page artifacts are then added in profiles. This avoids the need to keep both lists in sync (the lists are already out of sync).

The changes are currently just for `kie-wb`. Once (and if) we agree on the final look, I will also do the same for `kie-drools-wb`.

@manstis please take and let me know what you think.