Disable value_str

value_str is a way to add text on a button without a separate label.
This saves having an extra label object, but uses more memory and is
removed in LVGL8

src/displayapp/screens/Metronome.h

@@ -31,6 +31,7 @@ namespace Pinetime {
         lv_obj_t *bpmArc, *bpmTap, *bpmValue;
         lv_obj_t *bpbDropdown, *currentBpbText;
         lv_obj_t* playPause;
+        lv_obj_t* lblPlayPause;
 
         lv_task_t* taskRefresh;
       };