Revert "Only define screens after the config is loaded"

This reverts commit e6037b4738. It causes many
issues with non-trivial solutions that we first need to come up with. As a
middle ground, we will for now only emit signals if something changes after the
config is loaded.

See https://github.com/awesomeWM/awesome/issues/799.

awesome.c

@@ -643,6 +643,9 @@ main(int argc, char **argv)
     /* init atom cache */
     atoms_init(globalconf.connection);
 
+    /* init screens information */
+    screen_scan();
+
     /* do this only for real screen */
     ewmh_init();
     systray_init();
@@ -702,9 +705,6 @@ main(int argc, char **argv)
 
     xdgWipeHandle(&xdg);
 
-    /* init screens information */
-    screen_scan();
-
     /* scan existing windows */
     scan(tree_c);