Fix DoMoveWindow() to take into account the frame extent

Using "move" is correct for positioning, as it takes into account the
frame extent. Unfortunately, there is no corresponding API to set the
frame size. So we need to compute the effective client size and use
"resize". We can't use "setGeometry" for this purpose, since a widget's
"geometry" excludes the frame size. We would need to adjust the position
to go from frame to client coordinates. It's more straightforward to
simply have Qt do it with "move".

Closes https://github.com/wxWidgets/wxWidgets/pull/1349

src/qt/window.cpp

@@ -977,9 +977,19 @@ void wxWindowQt::DoMoveWindow(int x, int y, int width, int height)
     QWidget *qtWidget = GetHandle();
 
     qtWidget->move( x, y );
-    qtWidget->resize( width, height );
-}
 
+    // There doesn't seem to be any way to change Qt frame size directly, so
+    // change the widget size, but take into account the extra margins
+    // corresponding to the frame decorations.
+    const QSize frameSize = qtWidget->frameSize();
+    const QSize innerSize = qtWidget->geometry().size();
+    const QSize frameSizeDiff = frameSize - innerSize;
+
+    const int clientWidth = std::max(width - frameSizeDiff.width(), 0);
+    const int clientHeight = std::max(height - frameSizeDiff.height(), 0);
+
+    qtWidget->resize(clientWidth, clientHeight);
+}
 
 #if wxUSE_TOOLTIPS
 void wxWindowQt::QtApplyToolTip(const wxString& text)