tdf#104235: Qt5Frame: Set default icon

Set a default icon in Qt5Frame constructor, similar to how it's done
for the gtk and gtk3 VCL plugins in 'GtkSalFrame::InitCommon'.

Change-Id: I4e4e62ffa0eca6499f4752ff467d004cae68788f
Reviewed-on: https://gerrit.libreoffice.org/68450
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/qt5/Qt5Frame.cxx

@@ -161,6 +161,8 @@ Qt5Frame::Qt5Frame(Qt5Frame* pParent, SalFrameStyleFlags nStyle, bool bUseCairo)
     //m_aSystemData.pWidget = m_pQWidget;
     //m_aSystemData.nScreen = m_nXScreen.getXScreen();
     m_aSystemData.pToolkit = "qt5";
+
+    SetIcon(SV_ICON_ID_OFFICE);
 }
 
 Qt5Frame::~Qt5Frame()