Bug 887712 - [SMS/MMS] Make editable cursor element "zero width" until any user interaction

https://bugzilla.mozilla.org/show_bug.cgi?id=887712

- Set zero-width on editable placeholder until any interaction
- Remove zero-width during interactions
- Restore zero-width  on editable after interaction (signified by loss of focus by any means)

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>