Move all strings to xml files. Localization for EN, PL.

All strings are now stored in strings.xml files.
Localization is done for:
 * EN
 * PL

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