Better way to handle stoping subtitle edit mode.

Instead of handling focusout, we handle the mousedown event anywhere in
the document.  If the click is in the textarea, or the info tray, we
continue editing, otherwise we cancel.
Refactored the SubtitleItem code.

Instead of a single SubtitleItem class, we now distinguish between
subtitles that are currently stored in the DFXP and subtitles that are
in the process of being edited.

This is a cleaner version of the old fake subtitle code that the
timeline used and we can use it to update the trayinfo as we change
things.
Make the info tray update as you edit.

Refactored the code and combined infoTray/editText into the
CurrentEditManager class.  Have that use a draft subtitle that gets
updated as the user types.
Fix for info tray position after navigating to the next/prev subtitle