DesktopBackground: Make the child window invisible.

If the child window is visible, when running the Tutorial the wallpaper is not shown.
DesktopBackground: added get_dominant_color() static method.

Given a GdkPixbuf, this static method will return the
dominant color found in the image.

Method based on GetDominantColorOfImage() in linuxdeepin /
go_lib @ GitHub, committed by fasheng. Thank you!
https://github.com/linuxdeepin/go-lib/blob/master/gdkpixbuf/gdk_pixbuf_utils.c
Tutorial: finally fixed the 'grey background' issue.

We actually need the child window (thus this commits also reverts bbfe5ae), so we solved this
by setting the Tutorial background to transparent.