[ENHANCEMENT] structure tagging

This PR allows structure tags to be defined in the structure template
file. These tags can subsequently be used by other bundles.

__Tasks:__

- [x] test coverage
- [x] Reviewed by Dan
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none - helps with the search PR
| BC Breaks     | yes - get/setIndexName on structure has been removed
| Doc           | 

I checked this with @wachterjohannes it is because priority doesn't apply to tags such as those which we are using for the SearchBundle. By default all tags are assigned a priority of 1. We decided that its fine for things to have an equal priority.