Add MATE-Stripes Wallpapers

- Add desktop/MATE-Stripes-Dark.png semi-transparent PNG wallpaper
- Add desktop/MATE-Stripes-Light.png semi-transparent PNG wallpaper
- Add both files to desktop/Makefile.am
- Add wallpaper metadata for MATE-Stripes Wallpapers
Merge pull request #14 from hekel/master

Add MATE-Stripes Wallpapers