Add Cancel button to "Size Legend" dialog

because it's easier than hitting the cross button if you want to leave the dialog

Author: DelazJ

src/app/qgsdiagramproperties.cpp

@@ -1039,9 +1039,10 @@ void QgsDiagramProperties::showSizeLegendDialog()
   dlg.setLayout( new QVBoxLayout() );
   dlg.setWindowTitle( panel->panelTitle() );
   dlg.layout()->addWidget( panel );
-  QDialogButtonBox *buttonBox = new QDialogButtonBox( QDialogButtonBox::Help | QDialogButtonBox::Ok );
+  QDialogButtonBox *buttonBox = new QDialogButtonBox( QDialogButtonBox::Cancel | QDialogButtonBox::Help | QDialogButtonBox::Ok );
   connect( buttonBox, &QDialogButtonBox::accepted, &dlg, &QDialog::accept );
   connect( buttonBox, &QDialogButtonBox::helpRequested, this, &QgsDiagramProperties::showHelp );
+  connect( buttonBox, &QDialogButtonBox::rejected, &dlg, &QDialog::reject );
   dlg.layout()->addWidget( buttonBox );
   if ( dlg.exec() )
     mSizeLegend.reset( panel->dataDefinedSizeLegend() );