Patch to choose the vertical speed denominator

[Patrick Valsecchi]

Hi,

I've been told that some people (free divers) are loving their ft/s or 
m/s for vertical speeds.

The attached patch allows them to configure that.

While doing that, I was a bit surprised to have to change so many 
locations. Aren't MainWindow::writeSettings and 
PreferencesDialog::syncSettings a bit redondant?

CU

PS: I'm always testing only on Linux with Qt4
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