client: Decouple folder list entry DND from both main and GDK windows

Pass an instance of the owning `Sidebar.Tree` through to `Sidebar.Entry`
when calling `internal_drop_received` instead of a
`Application.MainWindow` to remove a bidi relationship between the two.

Use the high level GTK API for determining the DnD action in
`FolderList.Entry`'s implementation of this method, rather than attempt,
to divine it from keyboard modifiers via the associated GDK window,
since under Wayland that will be null.

Fixes #1163

src/client/folder-list/folder-list-folder-entry.vala

@@ -80,20 +80,29 @@ public class FolderList.FolderEntry :
         entry_changed();
     }
 
-    public bool internal_drop_received(Application.MainWindow main_window,
+    public bool internal_drop_received(Sidebar.Tree parent,
                                        Gdk.DragContext context,
                                        Gtk.SelectionData data) {
-        // Copy or move?
-        Gdk.ModifierType mask;
-        double[] axes = new double[2];
-        context.get_device().get_state(context.get_dest_window(), axes, out mask);
-        if ((mask & Gdk.ModifierType.CONTROL_MASK) != 0) {
-            main_window.folder_list.copy_conversation(folder);
-        } else {
-            main_window.folder_list.move_conversation(folder);
-        }
+        var handled = false;
+        var folders = parent as FolderList.Tree;
+        if (folders != null) {
+            switch (context.get_selected_action()) {
+            case MOVE:
+                folders.move_conversation(folder);
+                handled = true;
+                break;
 
-        return true;
+            case COPY:
+                folders.copy_conversation(folder);
+                handled = true;
+                break;
+
+            default:
+                // noop
+                break;
+            }
+        }
+        return handled;
     }
 
     public override int get_count() {

src/client/sidebar/sidebar-entry.vala

@@ -51,7 +51,7 @@ public interface Sidebar.DestroyableEntry : Sidebar.Entry {
 
 public interface Sidebar.InternalDropTargetEntry : Sidebar.Entry {
     // Returns true if drop was successful
-    public abstract bool internal_drop_received(Application.MainWindow main,
+    public abstract bool internal_drop_received(Sidebar.Tree parent,
                                                 Gdk.DragContext context,
                                                 Gtk.SelectionData data);
 }

src/client/sidebar/sidebar-tree.vala

@@ -1002,15 +1002,9 @@ public class Sidebar.Tree : Gtk.TreeView {
             return;
         }
 
-        bool success = false;
-
-        var main = get_toplevel() as Application.MainWindow;
-        if (main != null) {
-            success = targetable.internal_drop_received(
-                main, context, selection_data
-            );
-        }
-
+        bool success = targetable.internal_drop_received(
+            this, context, selection_data
+        );
         Gtk.drag_finish(context, success, false, time);
     }