Allow adding keys outside of the NLA strip, when Sync Length is
activated. Before this, an animator would have to exit tweak mode,
adjust the strip length, then enter tweak mode again.
Now the animator can freely insert and modify keys outside of the strip
bounds. They will never have to touch the strip bounds directly.
Reviewed By: sybren
Differential Revision: https://developer.blender.org/D7533
89ee260ef2