wibox: Set a proper back pixel

wibox: Set a proper back pixel

Currently wiboxes have their background set to "inherit parent pixmap". That
means that each time a wibox is e.g. resized, the X server sets the wibox'
content to the wallpaper and then immediately awesome redraws the wibox with the
proper background. This causes flicker when you e.g. resize clients which have a
titlebar.

With this patch, wiboxes get their proper background color for their "back
pixel" value. Now, instead of showing the wallpaper, the X server will fill the
complete wibox with its background color.

With this patch, the actual widgets will still flicker. Also, if the wibox has a
background image, this image obviously won't be used by the X server and we get
some flicker again. My next patch will address these issues.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Julien Danjou <julien@danjou.info>