Disable/enable start/stop button on start/stop measuring.

When measurign has been started then the start-button should be
disabled, and re-enabled when the measurign will be stopped.
The same for the stop-button.

Additional fix for displaying "N/A" as reference pressure when too
few samples had been measured and the measured has been stoped.

Signed-off-by: Janusz Kozerski <janusz.kozerski@gmail.com>
Refactoring of reset-reference-pressure action.

Optimization (info about mesuring reference pressure is displayed
      only one - after pushing the button)

After reset ref. pressure:
 * 0.0m high is displayed (only if sensor is running).
 * If sensor isn't running displayed high remains untouched.
 * If sensor isn't running N/A is displayed as a reference pressure value.

Signed-off-by: Janusz Kozerski <janusz.kozerski@gmail.com>