MapPart: Write 'hidden' XML attribute

Use negated logic to leverage our default behaviour of omitting false
values. With this change, files with all parts visible remain unchanged.
This fixes undesired changes to symbol sets, examples and test files.

Author: dg0yt

src/core/map_part.cpp

@@ -47,7 +47,7 @@ namespace literal
 	const QLatin1String objects("objects");
 	const QLatin1String object("object");
 	const QLatin1String count("count");
-	const QLatin1String visibility("visibility");
+	const QLatin1String hidden("hidden");
 }
 
 
@@ -96,7 +96,7 @@ void MapPart::save(QXmlStreamWriter& xml) const
 {
 	XmlElementWriter part_element(xml, literal::part);
 	part_element.writeAttribute(literal::name, name);
-	part_element.writeAttribute(literal::visibility, visible);
+	part_element.writeAttribute(literal::hidden, !visible);
 	{
 		XmlElementWriter objects_element(xml, literal::objects);
 		objects_element.writeAttribute(literal::count, objects.size());
@@ -115,8 +115,7 @@ MapPart* MapPart::load(QXmlStreamReader& xml, Map& map, SymbolDictionary& symbol
 
 	XmlElementReader part_element(xml);
 	auto part = new MapPart(part_element.attribute<QString>(literal::name), &map);
-	if (part_element.hasAttribute(literal::visibility))
-		part->visible = part_element.attribute<bool>(literal::visibility);
+	part->visible = !part_element.attribute<bool>(literal::hidden);
 
 	while (xml.readNextStartElement())
 	{