Add Stopwatch application

It can be started, stopped, and cleared. It also continues to run if
you leave the screen.

firmware/Application/Screens/MenuScreen.h

@@ -100,7 +100,7 @@ public:
     void render() {
         const struct font &font = default_font;
 
-        //m_driver.clear();
+        m_driver.clear();
 
         uint32_t x = 0;
         uint32_t y = 0;
@@ -124,6 +124,16 @@ public:
         m_driver.refresh();
     }
 
+    BSP::ReturnCode add_item(MenuScreenItem item) {
+        if (m_num_items ==  MAX_ITEMS) {
+            return BSP::ReturnCode::FAIL;
+        }
+
+        m_items[m_num_items++] = item;
+
+        return BSP::ReturnCode::OK;
+    }
+
     BSP::ReturnCode init() {
         return BSP::ReturnCode::OK;
     }
@@ -134,7 +144,6 @@ public:
 
     void enable() override {
         m_selected = 0;
-        m_driver.clear();
         render();
     }