Driver Keyframing: Some tweaks to make inserting keyframes on Driver F-Curves easier

Now, when trying to insert a keyframe on a driven property (using IKEY, or with
autokeying enabled), the keyframes will get created on the Driver's F-Curve
(instead of creating a new FCurve that goes into the active action, but will never
do anything). Furthermore, the x-value of the new keyframe will be the current
result of the driver expression.

Why/Motivations:
This way, it becomes easier to create corrective drivers, as you can position all
the targets the driver depends on, then adjust the driver value until it does what
you need, and then you keyframe that value to bake it into the Driver F-Curve
(in effect, "training" the computer how to behave in that case).

Usage Notes:
* In practice, that particular workflow is still quite clunky to achieve, due to some
  quirks of how the driver system and the UI widgets interact. Specifically, you'll
  need to disable/mute the driver before trying to edit the setting (to prevent the
  driver from immediately resetting the value - before even autokey fires!). However,
  if you're using the Graph Editor to preview/monitor/manage the keying process, you'll
  then want to re-enable the driver before changing the targets, so that you can see
  how much of a change you'll want to be applying!

* The warning about editing driver values may need to be disabled or selectively
  knocked out. I had it disabled while testing this functionality, but it's actually
  harmless in its current state (if just a bit annoying).

1 files changed, 1 insertions, 0 deletions

diff --git a/source/blender/editors/include/UI_interface.h b/source/blender/editors/include/UI_interface.h
index a2408812643..9aad340d2fb 100644
--- a/source/blender/editors/include/UI_interface.h
+++ b/source/blender/editors/include/UI_interface.h
@@ -495,6 +495,7 @@ bool    UI_but_active_drop_color(struct bContext *C);
 
 void    UI_but_flag_enable(uiBut *but, int flag);
 void    UI_but_flag_disable(uiBut *but, int flag);
+bool    UI_but_flag_is_set(uiBut *but, int flag);
 
 void    UI_but_drawflag_enable(uiBut *but, int flag);
 void    UI_but_drawflag_disable(uiBut *but, int flag);