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
Fix layout based decimal saperator on number pad.

Original fix submitted by Pierre Ossman.
See bugzilla Bug #652362 for more information.
Fix critical warnings when entering Financial Mode

Currency.number needs to be nullable.

https://bugzilla.gnome.org/show_bug.cgi?id=728826
Restore ans to previous value when undo.
Releasing 3.12.3
fixes segmentation fault error on repeatedly clicking undo