nunaliit2-js: Clean up show service. Use classes to track elements association to documents, requests for content and observe document updates.
nunaliit2-js: Clean up show service. Use classes to track elements
association to documents, requests for content and observe document
updates.

Issue #468
GCRC/nunaliit,GCRC/nunaliit,GCRC/nunaliit,GCRC/nunaliit,GCRC/nunaliit,GCRC/nunaliit,GCRC/nunaliit,GCRC/nunaliit