We don't need a public (but not exposed via headers) function in our ABI
to do gettext. Instead, we can just wrap _() to use g_dgettext() with a
predefined GETTEXT_PACKAGE.

Additionally, instead of requiring callers to check/initialize the
libpeas gettext textdomain, we can use a static constructor to set that
up once at startup.

Related !24

Fixes #35