rc.lua: Use the new screen requests instead of connect_for_each_screen

This will allow themes to better define their capabilities instead
of having to rely on the `rc.lua` `set_wallpaper` black box function.

awesomerc.lua

@@ -163,7 +163,7 @@ local tasklist_buttons = gears.table.join(
                                           end))
 
 -- @DOC_WALLPAPER@
-local function set_wallpaper(s)
+screen.connect_signal("request::wallpaper", function(s)
     -- Wallpaper
     if beautiful.wallpaper then
         local wallpaper = beautiful.wallpaper
@@ -173,16 +173,10 @@ local function set_wallpaper(s)
         end
         gears.wallpaper.maximized(wallpaper, s, true)
     end
-end
-
--- Re-set wallpaper when a screen's geometry changes (e.g. different resolution)
-screen.connect_signal("property::geometry", set_wallpaper)
+end)
 
 -- @DOC_FOR_EACH_SCREEN@
-awful.screen.connect_for_each_screen(function(s)
-    -- Wallpaper
-    set_wallpaper(s)
-
+screen.connect_signal("request::desktop_decoration", function(s)
     -- Each screen has its own tag table.
     awful.tag({ "1", "2", "3", "4", "5", "6", "7", "8", "9" }, s, awful.layout.layouts[1])