add command toBackground (#12)

available here : https://www.fully-kiosk.com/en/#rest and respond to the request here cgarwood/homeassistant-fullykiosk#64

Author: chpego

fullykiosk/__init__.py

@@ -81,6 +81,9 @@ async def stopSound(self):
 
     async def toForeground(self):
         await self.sendCommand("toForeground")
+    
+    async def toBackground(self):
+        await self.sendCommand("toBackground")
 
     async def startApplication(self, application):
         await self.sendCommand("startApplication", package=application)