[libcalamares] Start making InstanceDescription a stronger type

- Replace a map-of-strings with a class type.
- For now, doesn't compile.
- Intention is to construct from a YAML / QVariant from the
  *instances* list in `settings.conf`.

src/libcalamares/Settings.cpp

@@ -71,6 +71,10 @@ requireBool( const YAML::Node& config, const char* key, bool d )
 namespace Calamares
 {
 
+InstanceDescription::InstanceDescription( const QVariantMap& m )
+{
+}
+
 Settings* Settings::s_instance = nullptr;
 
 Settings*
@@ -134,17 +138,7 @@ interpretInstances( const YAML::Node& node, Settings::InstanceDescriptionList& c
                 {
                     continue;
                 }
-                QVariantMap instancesVListItemMap = instancesVListItem.toMap();
-                Settings::InstanceDescription instanceMap;
-                for ( auto it = instancesVListItemMap.constBegin(); it != instancesVListItemMap.constEnd(); ++it )
-                {
-                    if ( it.value().type() != QVariant::String )
-                    {
-                        continue;
-                    }
-                    instanceMap.insert( it.key(), it.value().toString() );
-                }
-                customInstances.append( instanceMap );
+                customInstances.append( InstanceDescription( instancesVListItem.toMap() ) );
             }
         }
     }

src/libcalamares/Settings.h

@@ -32,6 +32,12 @@
 namespace Calamares
 {
 
+class DLLEXPORT InstanceDescription
+{
+public:
+    InstanceDescription( const QVariantMap& );
+};
+
 class DLLEXPORT Settings : public QObject
 {
     Q_OBJECT
@@ -45,7 +51,6 @@ public:
 
     QStringList modulesSearchPaths() const;
 
-    using InstanceDescription = QMap< QString, QString >;
     using InstanceDescriptionList = QList< InstanceDescription >;
     InstanceDescriptionList customModuleInstances() const;