Allow custom extensions to render arbitrary things on the stage

[GarboMuffin]

Since custom extensions are based on <iframe> now, we could probably stack them on top of the canvas to allow some interesting possibilities.

[GarboMuffin]

#633 is the approach we will be going with. Custom extensions with that mechanism will be able to render whatever they want on the stage.

[David-Orangemoon]

I wrote an unsandboxed extension that does smth similar except it just overwrites whats on the stage this might be useful.