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This also fix a typo/bug when checking if a clip has already been
loaded.

This also fix a potential bug when using max end

Nothing else is saved, as we now use a QXmlStreamWriter. The file format
has changed.
Using forward declaration for class Media as soon as possible.

Fixed some slots name at the same time.

The import button is to be added when required.

Also removed old library widget ui file.

When previous button is clicked, the current view is directly deleted,
which avoid doing it in every ListViewController implementation

As it is a view, it doesn't interact directly with the MediaContainer,
but emit signals, and watches for MediaContainer changes to update
itself, thus (mostly) stopping this permanent mvc pattern rape.

Media aren't meant to be used anywhere, except when rendering. Clip can
now have subclips, we avoid using uuid when using Clip* is possible.
Correcting remaining crap in MediaListViewController and co.
View handle as many things as possible, but do not touch data.
ImportController doesn't have to do anything with deleting media. That's
the MediaContainer's job.
Probably many other things...

The constness difference was breaking renderers inheritance