Use compile-time Unicode string conversion

Thanks to this syntax introduced in C++11, this reduces the amount of work
that needs to be performed at run-time while making the code more terse.

editor/plugins/sprite_frames_editor_plugin.cpp

@@ -1282,7 +1282,7 @@ void SpriteFramesEditor::_update_library(bool p_skip_selector) {
 			// Frame is often saved as an AtlasTexture subresource within a scene/resource file,
 			// thus its path might be not what the user is looking for. So we're also showing
 			// subsequent source texture paths.
-			String prefix = String::utf8("┖╴");
+			String prefix = U"┖╴";
 			Ref<AtlasTexture> at = texture;
 			while (at.is_valid() && at->get_atlas().is_valid()) {
 				tooltip += "\n" + prefix + at->get_atlas()->get_path();