initial work for dynamically registering playable video extensions. #264

commiting here to discuss what to do with the blacklist and pref
clean up code for dynamic creation of importable video extensions

Conflicts:

components/mediacore/gstreamer/src/sbGStreamerMediacoreFactory.cpp
use two slashes for pref info like other lines
add preference defined blacklisting of audio and video extensions, re #264

Conflicts:
	app/preferences/songbird-prefs.js.in