do screen size better

src/gtkgui_helpers.py

@@ -27,9 +27,8 @@ _ = i18n._
 from common import gajim
 from common import helpers
 
-screen = gtk.gdk.Screen()
-screen_w, screen_h = screen.get_width(), screen.get_height()
-del screen
+screen_w = gtk.gdk.screen_width()
+screen_h = gtk.gdk.screen_height()
 
 def get_default_font():
 	''' Get the desktop setting for application font
@@ -180,4 +179,4 @@ def resize_window(window, w, h):
 		w = screen_w
 	if h > screen_h:
 		h = screen_h
-	window.resize(abs(w), abs(h))
\ No newline at end of file
+	window.resize(abs(w), abs(h))