Add Dutch (nl) translation.
Don't use the custom renders in the hotkeys editor on OS X

The standard controls aren't particularly usable, but the OS X
implementation of wxDVC doesn't actually support custom renderers and
not very usable is mildly better than entirely nonfunctional.
Don't invert horizontal scroll on the audio display on OS X

For some reason horizontal scroll comes pre-flipped.
Implement TransferToWindow in DoubleValidator. Closes #1606.