Materials: Add import for unimplemented fields

Adds import/export support for unimplemented material fields.

This includes reference source and URLs, and tag fields. Tag support
remains unimplemented in the editor, but is supported by a number
of materials in the FCMat files. They can be accessed in both C++
and Python.

src/Mod/Material/App/Materials.cpp

@@ -699,6 +699,19 @@ void Material::removeUUID(QSet<QString>& uuidList, const QString& uuid)
     uuidList.remove(uuid);
 }
 
+void Material::addTag(const QString& tag)
+{
+    auto trimmed = tag.trimmed();
+    if (!trimmed.isEmpty()) {
+        _tags.insert(trimmed);
+    }
+}
+
+void Material::removeTag(const QString& tag)
+{
+    _tags.remove(tag);
+}
+
 void Material::addPhysical(const QString& uuid)
 {
     if (hasPhysicalModel(uuid)) {
@@ -1327,6 +1340,12 @@ void Material::saveGeneral(QTextStream& stream) const
     if (!_reference.isEmpty()) {
         stream << "  ReferenceSource: \"" << MaterialValue::escapeString(_reference) << "\"\n";
     }
+    if (!_tags.isEmpty()) {
+        stream << "  Tags:\n";
+        for (auto tag : _tags) {
+            stream << "    - \"" << tag << "\"\n";
+        }
+    }
 }
 
 void Material::saveInherits(QTextStream& stream) const