[library_html5] Simplify JSEvent system. NFC

This change extracts a few helper function to make the JS event system
smaller and more internally consistent.

Firstly, we now use a single storage location for the event data pointer
used in event callback.  This is allocated on demand in
`allocateEventStruct`.  We then use a new `dispatchEvent` all the many
loction that previously duplicated the event dispatching logic.

In order to handler the 3 cases where we need to keep a copy of the
last dispatched event we also have two new helpers: `getCachedEvent`,
`cacheEvent`.  These allocate separate copies of the last generated
event, but only for the 3 event types where this is required.

Author: sbc100