Allow `Qt.labs.settings 1.0` to work with non-constant expressions.

When defining various settings for a qml project it might be useful to not set a standard value for width or height, but something that depends on a function. This could be useful if you want to account for pixel density of the various screens.

Settings {
        id: settings
        property alias x: mainwindow.x
        property alias y: mainwindow.y
        property alias width: Units.dp(800)
        property alias height: Units.dp(600)
    } 

Here Units.dp() returns a value depending on the screen it is currently being used, and as a result width or height will not function in my settings. Every time I open the app only information on x, and y position will be applied.