screen: Add a function to get the client preferred screen

lib/awful/screen.lua

@@ -12,7 +12,8 @@ local capi =
 {
     mouse = mouse,
     screen = screen,
-    client = client
+    client = client,
+    awesome = awesome,
 }
 local util = require("awful.util")
 local object = require("gears.object")
@@ -213,6 +214,15 @@ function screen.object.set_padding(self, padding)
     end
 end
 
+--- Return the currently screen for new clients.
+-- This is exactly the same as `awful.screen.focused` exept it avoids clients
+-- being moved when Awesome is restarted. This is used by the default `rc.lua`.
+-- @tparam client c A client
+-- @treturn screen The preferred screen.
+function screen.preferred(c)
+    return capi.awesome.startup and c.screen or screen.focused()
+end
+
 --- The defaults arguments for `awful.screen.focused`
 -- @tfield[opt=nil] table awful.screen.default_focused_args