Update a bunch of gettext stuff

Add support for a PURPLE_LOCALE_DIR environment variable so we can set the
locale directory in a dev environment.

Added purple_get_locale_dir() which will check the environment variable or
fallback to PURPLE_LOCALE_DIR.

Removed the old PACKAGE and PACKAGE_NAME constants and replaced them with
GETTEXT_PACKAGE which is much more self explanatory.

Testing Done:
Compiled with finch enabled and verified no issues.

Reviewed at https://reviews.imfreedom.org/r/2759/