Change the behaviour of circle select on FCurves:
- Before any key is touched, the behaviour is as it was (key included in
circle-select area → select it; otherwise → select entire curve)
- If any key has been touched, an internal option
(`use_curve_selection`) is flipped and selecting the entire curve is
disallowed.
This ensures that once a key changes selection state, it's no longer
possible to select the entire curve. This allows dragging over keys and
subsequently drag over a keyless part of the curve.
Reviewed By: RiggingDojo, troopy28
Maniphest Tasks: T101894
Differential Revision: https://developer.blender.org/D16307
c3391d537b