The Equalize Handles and Snap Keys operators would allow the user to
invoke them successfully even when they would have no effect due to
there not being any selected control points.
This patch makes it so that an error is displayed when these operators
are invoked with no control points are selected.
The reason this is in the `invoke` function is because it would be too
expensive to run this check in the `poll` function since it requires a
linear search through all the keys of all the visible F-Curves.
Reviewed By: sybren
Differential Revision: https://developer.blender.org/D16390
3836b6ff8c