Add support for changing the DS and DSi battery level

The DS battery level is configured via the SPI Power Management Device,
and the DSi's is configured via the I2C BPTWL. Add support for changing
these registers and add the "Power Management" dialog in the UI.

src/SPI.cpp

@@ -624,6 +624,9 @@ void Reset()
     RegMasks[4] = 0x0F;
 }
 
+bool GetBatteryLevelOkay() { return !Registers[1]; }
+void SetBatteryLevelOkay(bool okay) { Registers[1] = okay ? 0x00 : 0x01; }
+
 void DoSavestate(Savestate* file)
 {
     file->Section("SPPW");