Create z_index to define layer for Mobjects

[TonyCrane]

Closes #117

Related：
3b1b/manim#813
manim-kindergarten/manim@dde65b4
manim-kindergarten/manim@9467514

[PgBiel]

Regarding @eulertour's comment on getters: Imo, with the eventual usage of attrs, we should eventually transition into a general set (or edit, something like that) function for each class, allowing something like the example below, which allows for setting multiple attributes at the same time and is overall better:

mobj.set(x=5, y=6, z_index=50, ...)

The indiviudal set functions should be kept if they require more than one argument (although we could easily fix that by requiring a list on .set) or if they require any special work (in this case, the .set function would simply call the specific ones for you).

Anyway, this is better suited for its own issue. But I just thought about this