Improve heating interfaces

Make heating inherit from power and require min/max values for
thermostat's targetTemperature

libnymea/interfaces/heating.json

@@ -1,3 +1,4 @@
 {
-    "description": "The heating interface defines basic heating appliances."
+    "description": "The heating interface defines basic heating appliances.",
+    "extends": "power"
 }

libnymea/interfaces/thermostat.json

@@ -4,7 +4,9 @@
     {
       "name": "targetTemperature",
       "type": "double",
-      "unit": "DegreeCelsius"
+      "unit": "DegreeCelsius",
+      "minValue": "any",
+      "maxValue": "any"
     }
   ]
 }