This is part of #121565.
Renames the `insert_blank_frame` function to `insert_frame`.
Instead of returning a boolean for if the keyframe was created,
return a pointer to the drawing. This aligns more with how
we're using this API. After inserting a keyframe, it's common
to then e.g. write to the newly created drawing.
Also use sensible default parameter values for the duration
(default = implicit hold) and the keyframe type
(dafault = `BEZT_KEYTYPE_KEYFRAME`).
With this change, we're moving closer to the goal of
only allowing to create drawings by creating keyframes.
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