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

(cherry picked from commit 7e445c328e86f03cae8e0eed4dc6a742cacb9745)
Set min for cache_size to 100 MB

This is however not enforced in the code, it's just used for the
preferences dialog!

(cherry picked from commit f04c36b72d157d3e2886f145501358d9f2c77c8e)
Set cache_memory as int64

(cherry picked from commit 17b1046f4f2d951be748164ea0cc864da1f7ec36)