invert inheritance of legacy classes

This 'inverts' the inheritance of the following classes (makes the legacy classes inherit from the namespaced classes):

classes to convert:
- [x] `Zikula_ModUrl`
- [x] `Zikula_Hook_AbstractHandler` / `Zikula\Core\Hook\AbstractHookListener`
- [x] `Zikula_Hook_ValidationProviders` / `Zikula\Core\Hook\ValidationProviders`
- [x] `Zikula_Hook_ValidationResponse` / `Zikula\Core\Hook\ValidationResponse`
- [x] `Zikula_Response_DisplayHook` / `Zikula\Core\Hook\DisplayHookResponse`
- [x] `Zikula_DisplayHook` / `Zikula\Core\Hook\DisplayHook`
- [x] `Zikula_FilterHook` / `Zikula\Core\Hook\FilterHook`
- [x] `Zikula_ProcessHook` / `Zikula\Core\Hook\ProcessHook`
- [x] `Zikula_ValidationHook` / `Zikula\Core\Hook\ValidationHook`
- [x] add `@deprecated` tags to all legacy classes
- [x] testing

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | na
| Refs tickets  | #993, #1001
| Fixed tickets | #993 
| License       | MIT
| Doc PR        | na