Animation: Change active FCurve in box/circle/lasso select

Update the active FCurve when box/circle/lasso-selecting, such that after
the operator has finished the active curve is actually the last-operated-on
FCurve.

The operator already would deactivate the active FCurve. However, it would
not re-activate one any more, which caused the active keyframe to become
inactive.

Fix T81874: Box Select Keyframes doesnt set an active keyframe

Set the active keyframe on box/circle/lasso select in the graph editor.

For every FCurve that is influenced by the select operator that doesn't
have an active keyframe yet, the right-most keyframe is marked as the
active Keyframe.

Note that what is seen as the active keyframe for the graph editor also
depends on the active FCurve, which is not changed by box/circle/lasso
select.