Prevent label width from flickering rapidly in editor frametime panel

This also improves the background display by drawing a single background
for all 3 labels. This avoids visible corners between each label.

editor/plugins/node_3d_editor_plugin.h

@@ -255,6 +255,8 @@ private:
 	ViewportNavigationControl *look_control = nullptr;
 	ViewportRotationControl *rotation_control = nullptr;
 	Gradient *frame_time_gradient = nullptr;
+	PanelContainer *frame_time_panel = nullptr;
+	VBoxContainer *frame_time_vbox = nullptr;
 	Label *cpu_time_label = nullptr;
 	Label *gpu_time_label = nullptr;
 	Label *fps_label = nullptr;