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/displayapp/Apps.h

@@ -11,6 +11,7 @@ namespace Pinetime {
       FirmwareValidation,
       NotificationsPreview,
       Notifications,
+      Timer,
       FlashLight,
       BatteryInfo,
       Music,