Timer App (#355)

* built timer app

* Style improvements

* making sure buttons stay hidden when the app is reopened and reappear after the timer runs out

* more sensible calculations of time deltas. eliminated that mysterious scaling factor

* changing the timer icon

src/components/timer/TimerController.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <cstdint>
+#include "app_timer.h"
+#include "portmacro_cmsis.h"
+
+namespace Pinetime {
+  namespace System {
+    class SystemTask;
+  }
+  namespace Controllers {
+    
+    class TimerController {
+    public:
+      TimerController(Pinetime::System::SystemTask& systemTask);
+      
+      void Init();
+      
+      void StartTimer(uint32_t duration);
+      
+      void StopTimer();
+      
+      uint32_t GetTimeRemaining();
+      
+      bool IsRunning();
+    
+    private:
+      System::SystemTask& systemTask;
+      
+      static void timerEnd(void* p_context);
+  
+      TickType_t endTicks;
+      bool timerRunning = false;
+    };
+  }
+}