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This commit contains some crappy-things, such as a call to the mainloop
during a layout re-organisation and should be removed ASAP.

the MediaLibrary contains all of the media, but only gives access to the Clip, created upon the medias.
Cleaned metadata process: the clip doesn't appear inside of the mediaLibrary until the metadata process has been completed.
Removed junk/useless code

This reverts commit b99c1c6e.
There's a blocker which needs to be fixed first.

This works for adding/removing clip

It broked the uuid value on windows.