DataFile: added nested LocaleHelper class

In order to address issues with differently coded floating points in
strings provide a helper class which can be instantiated when loading
or saving data.
Song, InstrumentTrack: use DataFile::LocaleHelper

Always save projects and presets with default C locale in order to avoid
problems in the future. At the same time properly load floating point
strings which have been saved with different locale settings.

Closes #1051.
InstrumentTrack: fixed whats-this text