main: Add playlist page to GTK inspector

Merged Bastien Nocera requested to merge wip/hadess/playlist-inspector into master

Before this change, it was possible to visualise the playlist as a widget by recompiling totem while uncommenting a couple of lines of code.

Move this functionality to the GTK inspector by creating a module that will get loaded when the GTK inspector is.

Edited by Bastien Nocera

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