Enable single-window mode by default in projects

There are many issues with using multiple windows by default:

- Taking screenshots of a specific window will not capture subwindows.
  This also applies when recording a video using tools such as OBS.
- Subwindows may not behave correctly when fullscreen mode is enabled,
  especially if exclusive fullscreen is implemented in the future
  to decrease input lag on Windows.

"Subwindows" is quite general here, and also refers to dropdown menus
(e.g. those spawned by OptionButton) and Control tooltips.

Therefore, it's safer to embed subwindows by default in projects.
Multi-window mode remains the default in the editor.

main/main.cpp

@@ -2105,7 +2105,7 @@ bool Main::start() {
 		}
 #endif
 
-		bool embed_subwindows = GLOBAL_DEF("display/window/subwindows/embed_subwindows", false);
+		bool embed_subwindows = GLOBAL_DEF("display/window/subwindows/embed_subwindows", true);
 
 		if (single_window || (!project_manager && !editor && embed_subwindows)) {
 			sml->get_root()->set_embed_subwindows_hint(true);