Add forward declaration of QSettings

Its needed for Qt5

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Make dc_number defined when using it

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Translate using tr() to be Qt5-compatible

There are no utf8 in those strings, and we can translate them as
everything else with tr() instead.
QApplication::UnicodeUTF8-part is deprecated and removed in Qt5.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't devide by 0 if fO2 is 100%

Fixes #465

Initial-patch-by: Anton Lundin <glance@acc.umu.se>
Better-idea-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>