bug#53842: XFCE sets wallpapers to gc'd store paths.

[Brendan Tildesley]

Backgrounds in XFCE get set to their store path that over time get gc'd killing 
the background. Perhaps somehow making them point to 
/run/current-system/profile/share/backgrounds/xfce somehow would be better.

/home/b/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml
22:          <property name="last-image" type="string" 
value="/gnu/store/fmds2rcqqxmiq1ipk6wqbf8x68ks93pp-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
27:          <property name="last-image" type="string" 
value="/gnu/store/sc0lz41k6l41yyyrgh8k3x9cyvxwn418-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
32:          <property name="last-image" type="string" 
value="/gnu/store/gym3674d636cxlk1jxgwqn3qv55hm99d-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
37:          <property name="last-image" type="string" 
value="/gnu/store/gym3674d636cxlk1jxgwqn3qv55hm99d-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
60:          <property name="last-image" type="string" 
value="/gnu/store/fmds2rcqqxmiq1ipk6wqbf8x68ks93pp-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
65:          <property name="last-image" type="string" 
value="/gnu/store/sc0lz41k6l41yyyrgh8k3x9cyvxwn418-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
70:          <property name="last-image" type="string" 
value="/gnu/store/gym3674d636cxlk1jxgwqn3qv55hm99d-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
75:          <property name="last-image" type="string" 
value="/gnu/store/gym3674d636cxlk1jxgwqn3qv55hm99d-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>
80:          <property name="last-image" type="string" 
value="/gnu/store/l8psafxs8fhniyi0y6qycjv25bxm7pq6-xfdesktop-4.16.0/share/backgrounds/xfce/xfce-verticals.png"/>