Cache Tags for navigation and snippet area

[wachterjohannes]

Navigation and Snippet-Area should add Cache Tags to be invalidated when something changed there.

Following tags should be added (and also invalidated):

• Navigation context (invalidated when a page adds or remove that context, ...)
• Snippet Key (invalidated when a snippet with key was saved, the template was changed, ...)

[wachterjohannes]

fixed by https://github.com/sulu/SuluHeadlessBundle/pull/112