fix appending "Not supported" multiple times to label

The term "Not supported" is not appended to the label when it is already present.
The term is removed when no longer applicable.

src/app/qgsvectorlayerproperties.cpp

@@ -435,16 +435,24 @@ void QgsVectorLayerProperties::syncToLayer( void )
   mSimplifyDrawingGroupBox->setChecked( simplifyMethod.simplifyHints() != QgsVectorSimplifyMethod::NoSimplification );
   mSimplifyDrawingSpinBox->setValue( simplifyMethod.threshold() );
 
+  QString remark = QString( " (%1)" ).arg( tr( "Not supported" ) );
   if ( !( layer->dataProvider()->capabilities() & QgsVectorDataProvider::SimplifyGeometries ) )
   {
     mSimplifyDrawingAtProvider->setChecked( false );
     mSimplifyDrawingAtProvider->setEnabled( false );
-    mSimplifyDrawingAtProvider->setText( QString( "%1 (%2)" ).arg( mSimplifyDrawingAtProvider->text(), tr( "Not supported" ) ) );
+    if ( !mSimplifyDrawingAtProvider->text().endsWith( remark ) )
+      mSimplifyDrawingAtProvider->setText( mSimplifyDrawingAtProvider->text().append( remark ) );
   }
   else
   {
     mSimplifyDrawingAtProvider->setChecked( !simplifyMethod.forceLocalOptimization() );
     mSimplifyDrawingAtProvider->setEnabled( mSimplifyDrawingGroupBox->isChecked() );
+    if ( mSimplifyDrawingAtProvider->text().endsWith( remark ) )
+    {
+      QString newText = mSimplifyDrawingAtProvider->text();
+      newText.chop( remark.size() );
+      mSimplifyDrawingAtProvider->setText( newText );
+    }
   }
 
   // disable simplification for point layers, now it is not implemented