lbcolorcorrect: Add a doneLocating property to geolocator

As seen on !3487 comments by @ratijas the doneLocating property inside kcms/nightcolor/main.qml file is messy. Currently it simply checks whether the location is (0, 0) and if it is it returns false. This is not a good way to define it and feels hacky. So I have created a doneLocating property in geolocator which helps to check for changes. We can simply call locator.doneLocating to check for whether the locator is done or not.

Current implementation in main.qml:

readonly property bool doneLocating: locator !== undefined && !(locator.latitude == 0 && locator.longitude == 0)