Create <office:meta> only on demand (and merge MetadataManager into OdfContainer)

(BTW, MetadataManager was instantiated per OdfContainer class, not per instance, did I review that? :) )

Merged MetadataManager, because creating the <office:meta> on demand would have needed access to  internal helper method of OdfContainer (setChild(...)) so given that MetadataManager does not do that much currently (besides caching metadata in a separate JS map, but noone uses that ATM) it seemed less complex to just merge the few used calls into the API of OdfContainer. 

Counterproposal for #237 :)