statementview/snaklistview refactoring

Removed redundant code from statementview and snaklistview and moved some documentation.
The statementview widget does not trigger its own API request saving a (new) reference
anymore. The statementview-specific code reacting on adding a new reference has been
moved to the "afterstopediting" event handling.

Change-Id: I4b6ccc8873273c8b319a129980bf745185569679
Merge "statementview/snaklistview refactoring"