Added getters and notifications to allow clients of the loaders (DicomStructureSetLoader, OrthancSeriesVolumeProgressiveLoader and OrthancMultiframeVolumeLoader) to know when the loading is finished + added ability for SDL event loop to execute a callback repeatedly (used to check the view loading state)

{
    "include" : [
        "../../Platforms/Wasm/stone-framework-loader.ts",
        "../../Platforms/Wasm/wasm-application-runner.ts",
        "../../Platforms/Wasm/wasm-viewport.ts"
    ]
}