wallpaper: if previous advanced basic apply follows advanced setting.

Summary:
Previously if you had different wallpapers on different screens
then came back to the settings and changed the wallpaper ALL
screens would be set, and the painstaking work of setting
various wallpapers across desktops/screens is lost instantly.

This patch avoids this annoyance.

Reviewers: raster, zmike!, devilhorns

Reviewed By: raster

Subscribers: cedric

Tags: enlightenment-git

Differential Revision: https://phab.enlightenment.org/D7141