Draft: Add Firefox plugin
This allows us to build the Firefox OpenH264 plugin as part of the noopenh264 project, so we can have the Firefox plugin installed by default on Fedora. (It's disabled by default, though, because noopenh264 is part of the freedesktop-sdk project, which does not need it.)
This is ready for review, but let's wait to see if Kalev agrees before landing.