Show battery percentage on home screen (#167)

## Summary

* Show battery percentage on home screen
  * Moved battery rendering logic into shared ScreenComponents class
* As discussed
https://github.com/daveallie/crosspoint-reader/discussions/155

src/ScreenComponents.h

@@ -0,0 +1,8 @@
+#pragma once
+
+class GfxRenderer;
+
+class ScreenComponents {
+ public:
+  static void drawBattery(const GfxRenderer& renderer, int left, int top);
+};