Disallows moving desktop icons, basically force-enables the "positions locked" option

• Verified that the checkbox in settings and the context menu was checked and disabled
• Verified that it didn't touch the stored config so disabling the restriction would restore movable icons if it was enabled before
• Verified that changing the setting works if restriction isnt enabled

The docs also mention "renaming" and "removing" icons but this should be done by making Desktop folder read-only, which FolderView also doesn't handle very well. Dolphin, for instance, disables renaming and deleting right away whereas Folder View allows and then fails when doing so. Will fix that separately.