screen: Move the "added" signal from CAPI to Lua.

When the screens are created from the viewport in Lua, the signal is
sent too early and the DPI and outputs have not yet been added. This
cause the `connect_for_each_screen` callbacks to be called with a
partially initialized screen object. It also causes the drawables to be
repainted too early.

CAPI now emits "_added" and "awful.screen" takes care of emitting
"added".

lib/awful/screen.lua

@@ -937,11 +937,12 @@ require("awful.screen.dpi")(screen, data)
 
 -- Set the wallpaper(s) and create the bar(s) for new screens
 
-capi.screen.connect_signal("added", function(s)
+capi.screen.connect_signal("_added", function(s)
     -- If it was emited from here when screens are created with fake_add,
     -- the Lua code would not have an opportunity to polutate the screen
     -- metadata. Thus, the DPI may be wrong when setting the wallpaper.
     if s._managed ~= "Lua" then
+        s:emit_signal("added")
         s:emit_signal("request::desktop_decoration")
         s:emit_signal("request::wallpaper")
     end

lib/awful/screen/dpi.lua

@@ -160,6 +160,9 @@ function module.create_screen_handler(viewport)
 
     s:emit_signal("request::desktop_decoration")
     s:emit_signal("request::wallpaper")
+
+    -- Will call all `connect_for_each_screen`.
+    s:emit_signal("added")
 end
 
 function module.remove_screen_handler(viewport)

objects/screen.c

@@ -949,7 +949,7 @@ screen_added(lua_State *L, screen_t *screen)
     screen->workarea = screen->geometry;
     screen->valid = true;
     luaA_object_push(L, screen);
-    luaA_object_emit_signal(L, -1, "added", 0);
+    luaA_object_emit_signal(L, -1, "_added", 0);
     lua_pop(L, 1);
 }