Manage time constraints between clips

Registered by Pierre Nerzic

It could be useful to have a system of time constraints between clips :
- to ease time positionning of several clips : just have to say that this one will start immediately after another one,
- to allow the displacement of a former clip: the constrained clips will move too. This is the major use I see : when changing the duration of a clip in the beginning of a movie, I had to move manually every successive clips and transitions. I found it very painful.

These constraints could be expressed as :
- the beginning or the end of "this" clip equals the end or the beginning of that other clip plus or minus a fixed time delta.
We could also imagine more complex constraints, which could be useful to align a title (or subtitles): start of this = min( start of several other clips ), end of this = max( end of several other clips )...
I don't know how to deal with duration. I only see a use for start and finishing time.

Obviously, a check has to be made to not allow the creation of loops in the network of constraints.

It could be easy to specify these constraints in the properties window, at the "position on the timeline", "duration",... widgets. I would see a button drawn as a chain near the widget. This button opens a popup window which title is the name of the property to be constrained, then shows the list of all the clips present in the timeline (except those who can drive to a constraint loop) { question: does each clip in the timeline has a specific name even if it is present several times? }, one has to be chosen, a radio box with "start of clip" xor "end of clip" booleans to check, and finally a float "time shift" to add to the constraint. Two buttons below to confirm/add, or remove this constraint.

Constrained values could be displayed in a different color, and/or the constraint could be written as a text below the value. Constrained values cannot be edited manually.

At this step of thinking, I don't know whether one property must have only one or could have several constraints.

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