Bug 1461708 - part 7: Make EventStateManager::HandleMiddleClickPaste() dispatch ePaste event by itself r=smaug

This is preparation of the last patch.  Even if no editor is clicked with
middle button, we need to do:
- collapse Selection at the clicked point.
- dispatch "paste" event.

Therefore, HandleMiddleClickPaste() should dispatch ePaste event by itself
and each editor methods should have a bool argument which the caller wants
ePaste event automatically.

Note that Chromium dispatches "paste" event and pastes clipboard content
into clicked editor even if preceding "auxclick" event is consumed.
However, our traditional behavior is not dispatching "paste" event nor
pasting clipboard content.  Unless Chromium developer keeps their odd
behavior, we should keep our traditional behavior since our behavior is
conforming to DOM event model.

Differential Revision: https://phabricator.services.mozilla.com/D7854

--HG--
extra : moz-landing-system : lando