desktoppackage: warn on cancel if there are unsaved settings

When single click to preview wallpaper is enabled, it has the potential
of confusing users. When they click on the image they might think the
image is already applied and just close the window without hitting "apply".
This could be solved if a window popped up asking users to apply if
they try to close the window before applying, just like System Settings does.

CCBUG: 403384