Bezier Editor - add "Apply Function on Selected Keys"

Created by: puthre

Added Apply Function on Selected Key(s) feature on right click menu so you can now scale, move keys, set keys to a constant value and more easily create effects (like Bounce, Overshoot, etc) using the powerful godot Expression class. Functions can have parameters "time" and "value", "time" is the time of the key, "value" is the current value of the key.