Register keys for keypress only when needed

there seems to be an issue that with too many registered events some window
manager cannot recognize the device as input device with systemd 247.

PixlOne#166

src/logid/InputDevice.h

@@ -44,6 +44,7 @@ namespace logid
         explicit InputDevice(const char *name);
         ~InputDevice();
 
+        void registerKey(uint code);
         void moveAxis(uint axis, int movement);
         void pressKey(uint code);
         void releaseKey(uint code);