Bug 904990 - [Clock] Switching from Alarm Edit back to Clock with "Done" may lose the display

https://bugzilla.mozilla.org/show_bug.cgi?id=904990

- Store a timeout for hide's deferred class removal operation
- If the timeout occurs and is not cleared, clear the timeout and proceed with operation
- If show is called before hide's timeout is cleared, clear the timeout and cancel the operation

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