Use QWidget::update() rather than repaint() in wxClientDC

Qt documentation advises to avoid calling repaint() except when an
immediate update is required, i.e. it corresponds to wxWindow::Update().
Use update() instead to avoid many Qt warnings that appeared when
resizing the caret sample window, for example.

Closes https://github.com/wxWidgets/wxWidgets/pull/1178

src/qt/dcclient.cpp

@@ -106,7 +106,7 @@ wxClientDCImpl::~wxClientDCImpl()
             if ( !pict->isNull() && !widget->paintingActive() && !rect.isEmpty() )
             {
                 // only force the update of the rect affected by the DC
-                widget->repaint( rect );
+                widget->update( rect );
             }
             else
             {