7499/indexer

[schmengler]

This PR contains

• new indexer method reindexSlice() as nicer interface to reindex slices. The old way with additional parameters to reindex() still works, but has been deprecated
• an interface for the indexer. This is supposed to be used by other indexers (category indexer, cms indexer) as well to make it easier to use various indexers from a single place, like a CLI command. Public methods of the indexer that were only used by itself are not part of the interface and marked as deprecated in the class.
• a progress update dispatcher for the indexer. Clients can register progress handlers who receive updates about the indexer progress, which can be used to log or display the status during indexing
• PHPUnit requirement changed from 4 to 5 (4 to 6 requires too much change at once, 5 to 6 comes next)