AlphaFunction refactoring

AlphaFunction refactoring

Removed all the alpha functions from public api. Alpha functions can now be
defined using an enum to use one of the predefined alpha functions, a pointer,
to define a custom alpha function, or two Vector2 which are used for cubic bezier
alpha function.

The new API is used as follows:

//Using a predefined alpha function
animation.AnimateTo( property, value, AlphaFunction::EASE_IN_OUT );

//Using a custom alpha function
animation.AnimateTo( property, value, &my_alpha_function );

//Using a bezier alpha function
animation.AnimateTo( property, value, AlphaFunction(Vector2(0.0f,1.0f),Vector2(1.0f,0.0f) ) );

Change-Id: I4051c752d826770447923b6e9a3215f2575d6370