[Material Editor] Allow the "instancing" of material functions.

Oftentimes, when creating a material, we create functions which only use one function but with the inputs hooked up, for example when creating a landscape layer you would have a material function to make a layer, for example ‘MF_Landscape_Layer’, then you would have to create another material function ‘MF_Grass_Layer’, where the only thing used is the landscape layer function with some inputs hooked up with a constant.
This is cumbersome, especially when you have a lot of inputs and outputs, or when you have to change the main material function. So I propose material function “instancing”, where you create an instance of a material function, so that you can plug-in inputs with either constants, or leave some inputs as is to use in another material function or material, as well as leave exclude some outputs from showing. I think this can speed up many people’s UE4 Material workflow by a lot.