widget.background: Allow function as background image

There is already a hack into `awful.widget.common`. This system aim
to make the hack obselete while preserving the useful part.

I think this is also necessary to properly support SVG (with DPI
and resize).

Finally, Qt handle this using the QBrush concept, where you can have
programmatic patterns. Cairo doesn't have this concept, so there is no
"clean" way to have programmatic brushes.

lib/wibox/widget/background.lua

@@ -43,10 +43,14 @@ function background:draw(context, cr, width, height)
         cr:paint()
     end
     if self.bgimage then
+        if type(self.bgimage) == "function" then
+            self.bgimage(context, cr, width, height,unpack(self.bgimage_args))
+        else
             local pattern = cairo.Pattern.create_for_surface(self.bgimage)
             cr:set_source(pattern)
             cr:paint()
         end
+    end
 
 end
 
@@ -145,8 +149,12 @@ function background:set_shape_border_color(fg)
 end
 
 --- Set the background image to use
-function background:set_bgimage(image)
-    self.bgimage = surface.load(image)
+-- If `image` is a function, it will be called with `(context, cr, width, height)`
+-- as arguments. Any other arguments passed to this method will be appended.
+-- @param image A background image or a function
+function background:set_bgimage(image, ...)
+    self.bgimage = type(image) == "function" and image or surface.load(image)
+    self.bgimage_args = {...}
     self:emit_signal("widget::redraw_needed")
 end