[shotwell] Remove deprecated gdk_screen_get_monitor_at_window

[Jens Georg <jensgeorg src gnome org>]

commit b8309dd361b2fbfb284a30c31ffa87ad24d5f5f8
Author: Jens Georg <mail jensge org>
Date:   Fri Mar 30 16:59:03 2018 +0200

    Remove deprecated gdk_screen_get_monitor_at_window

 src/AppWindow.vala |    8 ++------
 1 files changed, 2 insertions(+), 6 deletions(-)
---
diff --git a/src/AppWindow.vala b/src/AppWindow.vala
index 2e8fd26..88a7934 100644
--- a/src/AppWindow.vala
+++ b/src/AppWindow.vala
@@ -115,12 +115,8 @@ public class FullscreenWindow : PageWindow {
     }
 
     private Gdk.Rectangle get_monitor_geometry() {
-        Gdk.Rectangle monitor;
-
-        get_screen().get_monitor_geometry(
-            get_screen().get_monitor_at_window(AppWindow.get_instance().get_window()), out monitor);
-
-        return monitor;
+        var monitor = get_display().get_monitor_at_window(AppWindow.get_instance().get_window());
+        return monitor.get_geometry();
     }
     
     public override bool configure_event(Gdk.EventConfigure event) {