Remove Millimetre usage

[miiizen]

This PR changes the following values to Spatium from Millimetre:

• Stem base length - keeps unit consistent with user length and line width
• Spacer gap - this was in Spatium already, but was being stored in a Millimetre type.
• Staff distance - same situation as above

This removes the need to rescale these values when changing spatium size, as the values are scaled when they are used rather than being stored after being scaled.

I have also moved the spacer layout code from its DOM class to TLayout.