Stop propagating keydown events that are handled by code hint list.

<del>Also switch to jQuery keydown event handler in KeyBindingManager so that event bubbling can be stopped by calling stopImmediatePropagation() in CodeHintList.</del>

This fixes issue #3975.

Update 1: We can't use jQuery keydown events since jQuery event object does not have "keyIdentifier" property that we need in KeyBindingManager. Instead of using jQuery keydown event, we just keep using native JavaScript event listener with event capturing "off" so that we can handle keydown events in bubbling phase.

Update 2: We can't handle keydown events in bubbling phase either. I just found out that Enter key may insert two lines when Emmet extension is installed since Emmet is inserting a formatted line break after CodeMirror inserts a line break. I'll be pushing another commit soon.