NightLight: refactored the code to make simpler

- using intensity instead of warmth
- animated color transition
- removed unecessary bindings and double properties
- using better icons to avoid confusion with brightness
- polished settings UI

Commons/Settings.qml

@@ -261,7 +261,6 @@ Singleton {
       // night light
       property JsonObject nightLight: JsonObject {
         property bool enabled: false
-        property real warmth: 0.0
         property real intensity: 0.8
         property string startTime: "20:00"
         property string stopTime: "07:00"