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This class isn't a QDialog anymore, and inherits WorkflowRender

There's some redrawing issues in rubberband mode, this will be fixed
at a later time.

Removed useless flags

(Removed dynamic_cast)

Conflicts:
	src/Workflow/ClipWorkflow.cpp
	src/Workflow/MainWorkflow.cpp
	src/Workflow/MainWorkflow.h
	src/Workflow/TrackWorkflow.cpp
	src/Workflow/TrackWorkflow.h
	src/gui/MainWindow.cpp
	src/renderer/WorkflowRenderer.cpp

This also solves a "lag" problem.

However, at the moment, the first frame isn't displayed... we need to move the position slider to get one...

IE you can pause the preview, but still change the position, and the render gets updated.

This is really much cleaner than the old versions...

Not needed anymore.
This reverts commit c7f780d7.

The crash occured when the first known position of the newly added clip
was exactly at the position of another one.