Re-add the progress bar to the calculator

Fixes #20583 - Raster calculator freezes the GUI with big rasters

... even if it is frozen while reding the raster,
at least it allows to abort the process later

src/app/qgisapp.cpp

@@ -5660,6 +5660,7 @@ void QgisApp::showRasterCalculator()
     QgsFeedback feedback;
     connect( &feedback, &QgsFeedback::progressChanged, &p, &QProgressDialog::setValue );
     connect( &p, &QProgressDialog::canceled, &feedback, &QgsFeedback::cancel );
+    p.show();
     QgsRasterCalculator::Result res = static_cast< QgsRasterCalculator::Result >( rc.processCalculation( &feedback ) );
     switch ( res )
     {
@@ -5706,6 +5707,7 @@ void QgisApp::showRasterCalculator()
                                    Qgis::Critical );
         break;
     }
+    p.hide();
   }
 }