Enable multiple canvases in single wasm module

- this commit renames CanvasId to CanvasSelector
  and forwards it to emscripten calls.
- made it so users can invoke `ProcessEvents` on the wasm
  interactor directly without calling `Start`. This is needed
  because the only way for users to process events on multiple
  wasm interactors is to call `emscripten_set_main_loop` with
  a lambda that invokes `ProcessEvents` on all interactors.
- removed the `canvas` key in the wasm module from
  examples and unit test html files.
- clean up wasm examples because the Module.canvas property is not required.
- simplify cmake setup in wasm examples