Provide safe API for detecting closing wxTipWindow

Previous API was ugly and suffered from a race condition as the window
could be closed before the constructor returned.

Provide a safer API using weak-pointer-like wxTipWindow::Ref class that
is guaranteed to stop being valid when the tip window is closed.

Also implement two-stage constructions for this class, as usual.

Closes #26070.

Closes #26071.

samples/dialogs/dialogs.cpp

@@ -763,10 +763,6 @@ MyFrame::MyFrame(const wxString& title)
     SetOwnBackgroundColour(m_canvas->GetBackgroundColour());
 #endif // wxUSE_INFOBAR
 
-#if wxUSE_TIPWINDOW
-    m_tipWindow = nullptr;
-#endif // wxUSE_TIPWINDOW
-
 #ifdef __WXMSW__
     // Test MSW-specific function allowing to access the "system" menu.
     wxMenu * const menu = MSWGetSystemMenu();
@@ -2858,14 +2854,13 @@ void MyFrame::OnShowTip(wxCommandEvent& WXUNUSED(event))
     }
     else
     {
-        m_tipWindow = new wxTipWindow
+        m_tipWindow = wxTipWindow::New
                           (
                             this,
                             "This is just some text to be shown in the tip "
                             "window, broken into multiple lines, each less "
                             "than 60 logical pixels wide.",
-                            FromDIP(60),
-                            &m_tipWindow
+                            FromDIP(60)
                           );
     }
 }