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.
Implemented the vera-color property that will be exported to the current running GTK+ theme.

(In a separate news, the color averaging algorithm has been changed and its now using code from
unity-greeter).

New configuration keys:
vera-color
vera-color-lock
Added 'vera-color' gtkmodule.

This module exports and updates the @vera-color property to the GTK+ theme.
Refactored the vera-color gtkmodule; added vera-color-enabled key to easily enable/disable the custom color.
vera-color: reset widgets also when disabling the vera-color support.
Merge branch 'master' into debian
vera-plugin-desktop.install: ship gtk module.
0.21.0-1
Updated gitignore.
Merge branch 'master' into debian