Example: demonstrate both adding and overriding Fess DI components

[marevol]

Summary

Make the example WebApp plugin demonstrate the two complementary ways a plugin
extends Fess's dependency-injection container, side by side:

1. Add a new component — ExampleHelper registered via the additive
   app++.xml merge convention.
2. Override a core component — CustomSystemHelper extends SystemHelper
   registered via the fess+systemHelper.xml per-component redefine convention.

Changes

• Add ExampleHelper + app++.xml (additive merge) in the unified package
  org.codelibs.fess.webapp.example.helper.
• Restore CustomSystemHelper + fess+systemHelper.xml (override) in the same
  package, so the original "replace an existing component" example is preserved.
• Sync the addDesignJspFileName design-JSP list in fess+systemHelper.xml to
  current Fess core (19 entries) so overriding systemHelper does not drop core
  design mappings (e.g. chat / busy / newpassword).
• Tests: ExampleHelperTest (additive) and CustomSystemHelperTest (override),
  each loading its own test DI config (test_app.xml / test_systemhelper.xml).
• Rewrite README.md to document both patterns and a corrected LastaDi
  merge/redefine conventions table.
• Fix the snapshot repository URL (central.sonatype.com).
• Align version references to 15.8.

Test plan

• mvn test passes (5 tests: ExampleHelperTest 3, CustomSystemHelperTest 2)
• mvn package produces a jar with Fess-WebAppJar: true and both DI files