Widgets should use custom language

[Beep6581]

Widgets such as Gtk::ColorButton (Preferences > General > Appearance > Crop mask color) and Gtk::FontButton (Preferences > General > Appearance > Main font) currently always use the system language as set by the locale, even when RawTherapee is set to use a custom language. The reason for this, according to GTK+ devs, is that RawTherapee should read the language from the options file and perform setlocale() in main() before anything else (so restart required), and not use g_setenv() as it currently does.

Simple way to test:

1. Set RawTherapee to use Deutsch or French.
2. Run RawTherapee using Italian (assuming you have an Italian locale on your system):

LC_ALL="it_IT.utf8" ./rawtherapee

[Floessie]

@Beep6581 I remember we had problems with setlocale() in #4319...