Implemented power management (#67)

- Implemented HAL for for power management
- Implemented the Power app (accessible via Settings app)
- Implemented status bar icon for battery status

boards/m5stack_core2/source/power.cpp

@@ -0,0 +1,34 @@
+#include "power.h"
+#include "M5Unified.hpp"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+static bool power_is_charging() {
+    return M5.Power.isCharging() == m5::Power_Class::is_charging;
+}
+
+static void power_set_charging_enabled(bool enabled) {
+    M5.Power.setBatteryCharge(enabled);
+}
+
+static uint8_t power_get_charge_level() {
+    uint16_t scaled = (uint16_t)M5.Power.getBatteryLevel() * 255 / 100;
+    return (uint8_t)scaled;
+}
+
+static int32_t power_get_current() {
+    return M5.Power.getBatteryCurrent();
+}
+
+Power core2_power = {
+    .is_charging = &power_is_charging,
+    .set_charging_enabled = &power_set_charging_enabled,
+    .get_charge_level = &power_get_charge_level,
+    .get_current = &power_get_current
+};
+
+#ifdef __cplusplus
+}
+#endif