Early bailout in calculator on empty formula
Add support for calculator formulas in conf

This is needed for cache_memory to be a nice looking value in the XML*.
If it turns out to be too slow we have to revert and remove the formula
from the XML file*.

* data/darktableconfig.xml.in
Fix overexposed borders to be float instead of int
Remove hardcoded default settings from the code

... these are all in the XML and default darktablerc now.