Dispatch trusted mouse events to content page (#751)

Dispatch trusted mouse events to content page (#751)

Some DOM APIs need to be called from the handler of a trusted event to
work. `requestFullscreen()` and `document.execCommand('copy')` are
examples of such API.

Before this patch, VimFx dispatched mouse events using
`element.dispatchEvent()` which had the event handlers for the events
running in untrusted context. This behavior prevented hint mode from
full-screening on Youtube and copying clone addresses on Github.

This patch uses XPCOM to dispatch mouse events as trusted events to the
content page.

See also:
https://www.w3.org/TR/uievents/#trusted-events - info from the spec
https://clipboardjs.com/ - before this patch, copying doesn't work