Adds a new function to insert a frame with a duration on a layer.
Also adds tests for the edge cases.
This introduces the concept of a `null-frame`.
It can be created with `GreasePencilFrame::null()` and checked for with
`is_null()`.
The purpose of a `null` frame is to indicate the end of whatever frame
comes before it. This way, the frames map does not need to store
the duration of frames. The duration is always implied by the distance
to the next frame.
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