[HTML5] Refactor JS, threads support, closures.

- Refactored the Engine code, splitted across files.
- Use MODULARIZE option to build emscripten code into it's own closure.
- Optional closure compiler run for JS and generated code.
- Enable lto support (saves ~2MiB in release).
- Can now build with tools=yes (not much to see yet).
- Dropped some deprecated code for older toolchains.
- Add onExit, and onExecute JS function.
- Add files drag and drop support.
- Add support for low precessor usage mode (via offscreen render, swap).

platform/javascript/SCsub

@@ -10,32 +10,56 @@ javascript_files = [
     "os_javascript.cpp",
 ]
 
-build = env.add_program(["#bin/godot${PROGSUFFIX}.js", "#bin/godot${PROGSUFFIX}.wasm"], javascript_files)
-js, wasm = build
+build_targets = ["#bin/godot${PROGSUFFIX}.js", "#bin/godot${PROGSUFFIX}.wasm"]
+if env["threads_enabled"]:
+    build_targets.append("#bin/godot${PROGSUFFIX}.worker.js")
+
+build = env.add_program(build_targets, javascript_files)
 
 js_libraries = [
-    "http_request.js",
+    "native/http_request.js",
 ]
 for lib in js_libraries:
     env.Append(LINKFLAGS=["--js-library", env.File(lib).path])
 env.Depends(build, js_libraries)
 
-js_modules = [
-    "id_handler.js",
+js_pre = [
+    "native/id_handler.js",
+    "native/utils.js",
 ]
-for module in js_modules:
-    env.Append(LINKFLAGS=["--pre-js", env.File(module).path])
-env.Depends(build, js_modules)
+for js in js_pre:
+    env.Append(LINKFLAGS=["--pre-js", env.File(js).path])
+env.Depends(build, js_pre)
 
-wrapper_start = env.File("pre.js")
-wrapper_end = env.File("engine.js")
-js_wrapped = env.Textfile("#bin/godot", [wrapper_start, js, wrapper_end], TEXTFILESUFFIX="${PROGSUFFIX}.wrapped.js")
+engine = [
+    "engine/preloader.js",
+    "engine/utils.js",
+    "engine/engine.js",
+]
+externs = [env.File("#platform/javascript/engine/externs.js")]
+js_engine = env.CreateEngineFile("#bin/godot${PROGSUFFIX}.engine.js", engine, externs)
+env.Depends(js_engine, externs)
+
+wrap_list = [
+    build[0],
+    js_engine,
+]
+js_wrapped = env.Textfile("#bin/godot", [env.File(f) for f in wrap_list], TEXTFILESUFFIX="${PROGSUFFIX}.wrapped.js")
 
 zip_dir = env.Dir("#bin/.javascript_zip")
-zip_files = env.InstallAs(
-    [zip_dir.File("godot.js"), zip_dir.File("godot.wasm"), zip_dir.File("godot.html")],
-    [js_wrapped, wasm, "#misc/dist/html/full-size.html"],
-)
+binary_name = "godot.tools" if env["tools"] else "godot"
+out_files = [
+    zip_dir.File(binary_name + ".js"),
+    zip_dir.File(binary_name + ".wasm"),
+    zip_dir.File(binary_name + ".html"),
+]
+html_file = "#misc/dist/html/full-size.html"
+in_files = [js_wrapped, build[1], html_file]
+if env["threads_enabled"]:
+    in_files.append(build[2])
+    out_files.append(zip_dir.File(binary_name + ".worker.js"))
+
+zip_files = env.InstallAs(out_files, in_files)
 env.Zip(
     "#bin/godot",
     zip_files,