Discard Changes feature : Enforce use of "Replace Frame"/"Add Frame"

Use Discard Changes feature : Make changes on the current frame, go to next frame [H] then go back on the frame you'd changed [G] => change disappeared. 

Impact : if you will to edit the frame you need to use "Replace/Add Frame" using [A] key before changing current frame.

How it is done ? When the user :

• Has loaded an animation
• Has played current animation
• Has selected another animation as current animation
• Has changed current animation's frame

Sets current frame to a deep copy of current frame.
When the user has set current frame index to nbFrame+1 (future frame), it sets the current frame to a deep copy of the animation's last frame

But ... Why enforcing the use of Replace/Add Frame button ?

It allow to discard changes that you made on current frame without having a CTRL+Z implementation (that would maybe be done in future update:) ). Discard change feature also reduce the impact of miss-manipulations (ie: having a blue vertex selected while clicking on a button and moving the mouse (drag event) => causes an arbitrary rotation) by resetting the copy to the lately added frame