Storing window geometry using Settings does not work correctly

https://stackoverflow.com/questions/31426528/saving-window-state-with-qml explains it, so I'll just quote part of that here:

The documentation gives the following method:

	Settings {
		property alias x: mainWindow.x
		property alias y: mainWindow.y
		property alias width: mainWindow.width
		property alias height: mainWindow.height

However that has three flaws:

1. It writes to the settings file continuously when you resize / move the window.
2. It doesn't remember whether the window is maximised or not (Notepad++ also suffers from this annoying flaw btw).
3. If you maximise the window it doesn't save the unmaximised geometry.

We should make this work out of the box, or provide a convenience type if that's not possible. It shouldn't be necessary to resort to such lengthy workarounds just to save a window's state.