[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/api.cpp

@@ -31,10 +31,12 @@
 #include "api.h"
 #include "core/engine.h"
 #include "javascript_eval.h"
+#include "javascript_tools_editor_plugin.h"
 
 static JavaScript *javascript_eval;
 
 void register_javascript_api() {
+	JavaScriptToolsEditorPlugin::initialize();
 	ClassDB::register_virtual_class<JavaScript>();
 	javascript_eval = memnew(JavaScript);
 	Engine::get_singleton()->add_singleton(Engine::Singleton("JavaScript", javascript_eval));