GTK3: Passed handle of hint win control to WS inside hint data pointer.

Closes #40331.

GTK3 WS needs to get that handle in order to retrieve the parent window
which need to be set as transient of the newly created popup window.

This is a required by GTK3 documentation as one can see in
  https://docs.gtk.org/gtk3/method.Window.set_transient_for.html

This patch needs to touch common LCL code, but `HintData` files does
not seem to be really used.

lcl/include/application.inc

@@ -848,6 +848,7 @@ begin
         Visible := False;
         Caption := '';
         AutoHide := False;
+        HintData := Pointer((FHintControl as TWinControl).Handle);
       end;
     end;
 

lcl/interfaces/gtk3/gtk3widgets.pas

@@ -814,6 +814,7 @@ type
 
   TGtk3HintWindow = class(TGtk3Window)
   private
+    procedure InitializeWidget; override;
   protected
     function CreateWidget(const {%H-}Params: TCreateParams):PGtkWidget; override;
   end;
@@ -7896,6 +7897,20 @@ begin
 
 end;
 
+procedure TGtk3HintWindow.InitializeWidget;
+var
+  ParentWidget: TGtk3Widget;
+begin
+  inherited;
+  with LCLObject as THintWindow do begin
+    if Assigned(HintData) then begin
+      ParentWidget := TGtk3Widget(HintData);
+      FWidget^.realize;
+      GetWindow^.set_transient_for(ParentWidget.GetWindow);
+    end;
+  end;
+end;
+
 { TGtk3Dialog }
 
 procedure TGtk3Dialog.SetCallbacks;