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This solves a dead lock (when rendering with an empty timeline)

Please note, this is only partially working. A bug in the backend
prevents the use of this functionnality.

IDs are now truly unique.

After some discussions we decided to remove this uid which didn't fit
well.

This reverts commit bd9b98fa.

Conflicts:

	src/Commands/Commands.hpp

An issue still remains in the undo/redo code and will be fixed asap.