Allow multiple DomEvent types per DOM event

It’s possible that different libraries provide DomEvent types for the
same DOM event (mainly HTML 5 events, e.g. transitionend). If that
happens only one library will work, as GWT only knows the last
registered DomEvent type. To fix this a one-to-many mapping between
DOM event name and DomEvent types has been introduced.

This patch also contributes to issue 8379 which aims to make the
event system more extendable.

Bug: issue 7698
Change-Id: Iedbac84881484849548e1829d61eb13a6c7d00e3
(cherry picked from commit a5327b8db8379737cf2775b784e8e7241a63cc59)
2 files changed