FEATURE #553 [SnippetBundle] Possiblity to show all snippet types

Default behavior when no snippet type is selected is now to show all snipet types.

_Tasks:__

- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | ywa
| Fixed tickets | #553 
| BC Breaks     | Default behavior is now to allow selection of all snippet types (instead of falling back to restricting on the default snippet type)
| Doc           | https://github.com/sulu-cmf/docs/pull/12