Proper "enter" key handling for entity selector's "more" link

Implemented proper "enter" key handling for suggester widget's custom items. Additionally,
event propagation is prevented for the entity selector's "more" link since it would cause
unrelated action. Furthermore, no string auto-completion is performed when fetching "more"
results since the "replace" operation would trigger another API call.
Setting proper focus will be done in another change set.

- patch set 2: rebase
- patch set 3: using correct offset
- patch set 4: updated documentation

Change-Id: I3e7f36666929c7cdf4c698553ea90f8be14e2117