Add QML plugin with ability to save window state declaratively

Jan Blackquill requested to merge work/janb/window-saver into master

This introduces an org.kde.kconfig import exposing a QML plugin with currently just one type: WindowStateSaver. It saves the state of the provided window under a given name. This makes relying on this functionality in QML more convenient and easier to conditionalise based on platform.

Merge request reports