TextOperationsWidget: mark text when choosing line

When the goto line widget is displayed, text is marked. Meaning you can
just change the line number.

src/widgets/textoperationswidget.cpp

@@ -495,6 +495,7 @@ void TextOperationsWidget::showGotoLine() {
     }
 
     showBottomWidget(gotoLineWidget);
+    gotoLineFormUi->numberSpinBox->selectAll();
     gotoLineWidget->setFocus();
     gotoLineFormUi->numberSpinBox->setFocus();
 }