Move hints used by Emscripten to window properties. This change will be necessary if multiple windows for Emscripten will be supported in the future.
- Added Window Create Property SDL_PROP_WINDOW_CREATE_EMSCRIPTEN_CANVAS_ID
- Added Window Create Property SDL_PROP_WINDOW_CREATE_EMSCRIPTEN_KEYBOARD_ELEMENT
- Use hint SDL_HINT_EMSCRIPTEN_CANVAS_SELECTOR as override to SDL_PROP_WINDOW_CREATE_EMSCRIPTEN_CANVAS_ID
- Use hint SDL_HINT_EMSCRIPTEN_KEYBOARD_ELEMENT as override to SDL_PROP_WINDOW_CREATE_EMSCRIPTEN_KEYBOARD_ELEMENT
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