[HTML5] Add JavaScriptToolsEditorPlugin.

A new editor plugin, specific to HTML5, that provide some extra features
needed to make the editor usable on that platform.

For now, it adds a "Download project sources" option in the "Tool" menu,
so the user can download the work done as a zip file (from the browser
storage).

platform/javascript/api/javascript_tools_editor_plugin.h

@@ -0,0 +1,32 @@
+#ifndef JAVASCRIPT_TOOLS_EDITOR_PLUGIN_H
+#define JAVASCRIPT_TOOLS_EDITOR_PLUGIN_H
+
+#if defined(TOOLS_ENABLED) && defined(JAVASCRIPT_ENABLED)
+#include "core/io/zip_io.h"
+#include "editor/editor_plugin.h"
+
+class JavaScriptToolsEditorPlugin : public EditorPlugin {
+	GDCLASS(JavaScriptToolsEditorPlugin, EditorPlugin);
+
+private:
+	void _zip_file(String p_path, String p_base_path, zipFile p_zip);
+	void _zip_recursive(String p_path, String p_base_path, zipFile p_zip);
+
+protected:
+	static void _bind_methods();
+
+	void _downalod_zip(Variant p_v);
+
+public:
+	static void initialize();
+
+	JavaScriptToolsEditorPlugin(EditorNode *p_editor);
+};
+#else
+class JavaScriptToolsEditorPlugin {
+public:
+	static void initialize() {}
+};
+#endif
+
+#endif // JAVASCRIPT_TOOLS_EDITOR_PLUGIN_H