Handle localized radix characters on command line

We were accounting for the locale's thousands separator, but not the
radix char (decimal point). For example:

export LC_NUMERIC=de_DE.UTF-8
gnome-calculator --solve 2.000,5 - ,2

The result should be 2.000,3 not an error.

https://bugzilla.gnome.org/show_bug.cgi?id=730905