Implement GetSizeFromTextSize() for wxSpinCtrl.

Implement it for the native MSW and GTK versions and the generic one used in
the other ports and also for wxSpinCtrlDouble under MSW.

Also test this function in the spin page of the widgets sample.

Closes #14840.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72995 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/widgets/widgets.cpp

@@ -875,6 +875,10 @@ void WidgetsFrame::OnSetFont(wxCommandEvent& WXUNUSED(event))
         (*it)->SetFont(m_font);
         (*it)->Refresh();
     }
+
+    // The best size of the widget could have changed after changing its font,
+    // so re-layout to show it correctly.
+    page->Layout();
 #else
     wxLogMessage(wxT("Font selection dialog not available in current build."));
 #endif