[partition] Don't signal changes from model before they're done

- clear() signals modelReset(), which is true, but inconvenient
   when we do a bunch of changes afterwards. Block signals,
   and rely on own signaling when all of the changes are done.
 - Keep blocking signals while updating the model, since the row
   appends otherwise trigger a change in the connected combo box.

src/modules/partition/core/BootLoaderModel.cpp

@@ -67,6 +67,8 @@ BootLoaderModel::createMbrItems()
 void
 BootLoaderModel::update()
 {
+    beginResetModel();
+    blockSignals( true );
     clear();
     createMbrItems();
 
@@ -111,6 +113,8 @@ BootLoaderModel::update()
             createBootLoaderItem( tr( "Do not install a boot loader" ), QString(), false )
         );
     }
+    blockSignals( false );
+    endResetModel();
 }