[shotwell] Fix (and comment) a deprecated call

[Jens Georg <jensgeorg src gnome org>]

commit 9502b97c1eeb62cc4d2d53befa1664cd6188c896
Author: Jens Georg <mail jensge org>
Date:   Wed Feb 19 21:37:09 2020 +0100

    Fix (and comment) a deprecated call

 src/editing_tools/EditingTools.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/editing_tools/EditingTools.vala b/src/editing_tools/EditingTools.vala
index e2c82565..9c6ee5e9 100644
--- a/src/editing_tools/EditingTools.vala
+++ b/src/editing_tools/EditingTools.vala
@@ -87,7 +87,8 @@ public abstract class EditingToolWindow : Gtk.Window {
     }
 
     public override void realize() {
-        set_opacity(Resources.TRANSIENT_WINDOW_OPACITY);
+        // Force the use of gtk_widget_set_opacity; gtk_window_set_opacity is deprecated
+        ((Gtk.Widget) this).set_opacity(Resources.TRANSIENT_WINDOW_OPACITY);
         
         base.realize();
     }