Keep the terminal number out of the element properties

The properties window builds a row for every element info key, so it
also offered an editable "Terminal number" row -- both for one element
and for several at once. The number is not a property of the element
but derived from its label: typing there stored a value nothing reads,
and the export has no column for it either (previous commit).

Both windows skip the key now, the way they skip exclude_from_bom.
The key itself stays an element info key, so the element editor keeps
offering %{terminal_number} as a text variable.

The test builds both windows for a terminal of the fixture and looks
at the rows they offer.

sources/ui/multielementinfowidget.cpp

@@ -496,6 +496,9 @@ void MultiElementInfoWidget::buildInterface()
 	QStringList keys = QETInformation::elementInfoKeys();
 		//It has its own check box above, see ElementInfoWidget::buildInterface
 	keys.removeAll(EXCLUDE_FROM_BOM);
+		//Not an editable property but derived from the label, see
+		//ElementInfoWidget::buildInterface
+	keys.removeAll(QETInformation::ELMT_TERMINAL_NUMBER);
 
 	int line = 0;
 	for (const QString &key : std::as_const(keys))