Add a V-Sync editor setting

The editor setting makes it possible to tweak V-Sync status
independently of the project setting.

Use cases:

- Decrease input lag and increase editor responsiveness when editing
  a project that has V-Sync enabled.
- Avoid tearing when editing a project that has V-Sync disabled.

editor/editor_node.cpp

@@ -345,6 +345,11 @@ void EditorNode::shortcut_input(const Ref<InputEvent> &p_event) {
 	}
 }
 
+void EditorNode::_update_vsync_mode() {
+	const DisplayServer::VSyncMode window_vsync_mode = DisplayServer::VSyncMode(int(EDITOR_GET("interface/editor/vsync_mode")));
+	DisplayServer::get_singleton()->window_set_vsync_mode(window_vsync_mode);
+}
+
 void EditorNode::_update_from_settings() {
 	_update_title();
 
@@ -758,6 +763,7 @@ void EditorNode::_notification(int p_what) {
 		} break;
 
 		case EditorSettings::NOTIFICATION_EDITOR_SETTINGS_CHANGED: {
+			_update_vsync_mode();
 			FileDialog::set_default_show_hidden_files(EDITOR_GET("filesystem/file_dialog/show_hidden_files"));
 			EditorFileDialog::set_default_show_hidden_files(EDITOR_GET("filesystem/file_dialog/show_hidden_files"));
 			EditorFileDialog::set_default_display_mode((EditorFileDialog::DisplayMode)EDITOR_GET("filesystem/file_dialog/display_mode").operator int());
@@ -6268,6 +6274,8 @@ EditorNode::EditorNode() {
 
 	FileAccess::set_backup_save(EDITOR_GET("filesystem/on_save/safe_save_on_backup_then_rename"));
 
+	_update_vsync_mode();
+
 	// Warm up the surface upgrade tool as early as possible.
 	surface_upgrade_tool = memnew(SurfaceUpgradeTool);
 	run_surface_upgrade_tool = EditorSettings::get_singleton()->get_project_metadata("surface_upgrade_tool", "run_on_restart", false);