Add missing information to po translation files in GIMP and GTK.

Gettext 0.19 complaints and fails because of missing information in
translation file headers.
This commit adds missing "Language:", "Language-Team",
"Project-Id-Version" information.
OS X build: add missing step to README

Add step to create directory in preparations.
OS X build: fetch latest stable GTK 2.24 version

Fetch version 2.24.23, remove obsolete patch.