Add 'slice' as a property to get and set slice9 values

[Jhonnyg]

Added a new property to sprites for getting and setting slice9 parameters. Examples:

local values = go.get("#sprite", "slice")
local value = go.get("#sprite", "slice.x")
go.set("#sprite", "slice", vmath.vector4(1,2,3,4))
go.set("#sprite", "slice.x", 1337)
go.animate("#sprite", "slice", go.PLAYBACK_LOOP_PINGPONG, vmath.vector4(96, 96, 96, 96), go.EASING_INCUBIC, 2)
go.animate("#sprite", "slice.x", go.PLAYBACK_LOOP_PINGPONG, 96, go.EASING_INCUBIC, 1)

Fixes #8740