* Now a Cel has a z-index property to change the order of layers per frame
* A new doc::RenderPlan class can calculate the order of cels to be rendered
* z-index is saved as a int16_t in the .aseprite files
* This new field can be set/get from Lua with Cel.zIndex
As the "multiple windows" feature is still buggy (#3556) and we've
disabled it by default, it's nice to make this option more visible (in
the General section) so users reliant on this will find the switch
quickly.
