Clear text controls and labels when histogram is empty

Clear text controls and labels when histogram is empty

- Do not display default time (epoch) for text controls and
start/end labels when histogram is initialized or when last trace is
closed
- Prevent user update of text controls when histogram is empty
- Fix layout problem on time format update

Change-Id: I9285853b127e0dca0fa0c2035fea1c414390a759
Reviewed-on: https://git.eclipse.org/r/9273
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>
IP-Clean: Bernd Hufmann <bhufmann@gmail.com>
Tested-by: Bernd Hufmann <bhufmann@gmail.com>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>