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
appdata: Mark at least one of the screenshots as default

The largest one chosen was the default for the software center.
Fix Malformed expression on base change.
Restore ans to previous value when undo.
Updated Brazilian Portuguese translation
fixes segmentation fault error on repeatedly clicking undo
Don't create new fundamental types

The GObject documentation says we should never need to do this, so let's
trust that and just never do it. It can lead to at least one dangerous,
hard to debug issue:

https://bugzilla.gnome.org/show_bug.cgi?id=708130#c21
build: include tests in release tarball
Updated Turkish translation
build: fix typo in step definitions files list
Releasing 3.13.3
build: strip unneeded test env setting and fix test install