Refactor internal tag handling architecture

Currently most of the tag related functions are fragmented over the whole application. Only for the internal content tags most code is centralized in the editor_Models_Segment_InternalTag class.
Same concept should be used for all our internal tags (MQM, Term Tags).

One goal is to provide the same interface for all tag types:
mqm tags, term tags and content tags extend abstract tag.
The abstract tag class defines the interface for commonly used methods: strip, replace etc.

Another general class internal tag should provide a similar interface to malke bulk operations, example:
generalTag->strip(string, MQM|term) → this leaves content tags