Better ticks

Device History graphs will have adjustable number of x axis divisions, so that the tick marks show at nice/rounded values. The patch also fixes the bug of having the x axis divisions and the option in "Data length" combo box in disagreement after a restart when the previous setting was 1 day or 1 week. I change the data length options of 10 minutes, 2 hrs, 6 hrs to 30 minutes, 3 hrs, 8 hrs for the following reason: 10 minutes is too short, data logging happens only every 2-or-so minutes; usual battery life seems to be something around 3 or up to 8 hours, so I thought extending these ranges would help us see the whole battery cycle. The latter changes required some updates to the .po files. I used

sed -i '/^msg/ s/"2 /"3 /' *.po
sed -i '/^msg/ s/"6 /"8 /' *.po
sed -i '/^msg/ s/"10 /"30 /' *.po

to fix these wherever possible. Some (non-latin script) files will have to be updated manually by translators.