Separate WebUI module

When working only on WebUI/integration, inside new `webui` directory: `mvn clean process-test-resources`.
- Validates (XML wellformness) and minify XHTML files.
- Checks syntax, merges (generally 1 bundle per screen) and minifies JS and CSS files.
- Compresses previously checked/optimized files using GZIP, to be served by web server along with HTTP header `Content-Encoding: gzip`.

What's required by this UI to be implemented in REST controllers is described by JSON mockup in `webui/src/test/webapp`.

When implementing REST controllers, priorly:
- in `webui` module: `mvn -Ddhek.staging=true clean package`,
- make development web server serves `webui/target/web` as document root, with appropriate configuration to add `Content-Encoding: gzip` header for `*.gz.*` (see `readme.md`).