Changing background can hang apps

[es20490446e]

HOW TO REPRODUCE:

1. Install the app "Variety".
2. Configure it to change background every 1 minute.

RESULT:

• When the background changes the entire desktop lags, and videos and audio play freeze for a second.
• From time to time applications permanently freeze.

LIKELY FIX:

• Set the wallpaper changing as a background process, with low CPU priority.
• Each time you change wallpaper, Deepin places a copy of it in "~/Pictures/Wallpapers". Likely that's the most computational expensive operation.

RELEVANT DETAILS:

• Having a Wallpapers backup folder is also creating another compatibility issue with Variety.

[sonald]

@es20490446e plz send to https://github.com/linuxdeepin/developer-center/issues, this is not a problem of deepin-wm

[es20490446e]

Done here, thanks.