backend: Use constructor to setup gettext

To ensure the translation data are loaded from the correct prefix even
if libintl and gnome-online-accounts are installed in different
prefixes, we must call bindtextdomain and bind_textdomain_codeset
before translating messages.

To keep the code clean, gconstructor.h from GLib is copied to support
running code when the library is loaded, so we can have a constructor
to call gettext functions.

https://bugzilla.gnome.org/show_bug.cgi?id=760440