fix #49: make debug screen show real data

* Break out debug screen to a separate class and make it thread-safe.
* Break out power state to a separate class.
* Show battery voltage, charging & USB status on debug screen.
* Show GPS lock / no lock
* Fix an off-by-one that I introduced earlier in `drawRows`.

src/power.h

@@ -0,0 +1,18 @@
+#pragma once
+
+namespace meshtastic
+{
+
+/// Describes the state of the power system.
+struct PowerStatus {
+    /// Whether we have a battery connected
+    bool haveBattery;
+    /// Battery voltage in mV, valid if haveBattery is true
+    int batteryVoltageMv;
+    /// Whether USB is connected
+    bool usb;
+    /// Whether we are charging the battery
+    bool charging;
+};
+
+} // namespace meshtastic