wasm: move event dispatcher implementation

Move the majority of the implementation from from
qeventloop.cpp to qhtml5eventdispatcher.cpp. Keep
the parts that handle nested event loops and app
exit since they require access to QEventLoop internals.

Change-Id: Ica344fdf7b3b7777985f7217e0c781580ec5e5b4
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>