Add a native for scaleform callbacks

This could be very useful for people who develop/work with scaleform and could make working with vanilla UI more easy.
Something like this:

Scaleform code example:

// is 1"SET_SOME_INT_VARIABLE",, 69);

Script example:

-- first param is name, second param is what type of event we're listening to
-- and third param is the callback function

local SCRIPT_TYPE = 1
local myVar = -1

AddScaleformCallbackHandler('SET_SOME_INT_VARIABLE', SCRIPT_TYPE, function(someIntVariable)
myVar = someIntVariable
print("myVar is now", myVar)