Updated version of media player plugin

I found this abandonned plugin that enables to play media files from Picard.
It was originally written by Gary van der Merwe.

I did a bit of cleanup, __it is not yet ready to be merged, but i would like a bit of feedback about it__, mostly because it requires slight changes in Picard main code.

Original code is from https://code.launchpad.net/~garyvdm/picard/player