[welcomeq] Fix build with clang

- can't convert lambda-with-captures to a function pointer (Clang 9)
- instead, use a context property .. QmlViewStep already sets a
  "config" property with the Config object, but WelcomeQ wants it
  as another name as well.
- this avoids registering the Welcome object across all QML pages,
  as well.

NOTE: needs to have the QML adjusted for this change.

src/modules/welcomeq/WelcomeQmlViewStep.cpp

@@ -51,7 +51,6 @@ WelcomeQmlViewStep::WelcomeQmlViewStep( QObject* parent )
 // 			 this,
 // 			 &WelcomeQmlViewStep::nextStatusChanged );
 //     emit nextStatusChanged(true);
-    qmlRegisterSingletonType< Config >( "io.calamares.module", 1, 0, "Welcome", [&](QQmlEngine*, QJSEngine*) -> QObject* { return m_config; } );
 }
 
 
@@ -158,6 +157,7 @@ WelcomeQmlViewStep::setConfigurationMap( const QVariantMap& configurationMap )
 	}
 
     Calamares::QmlViewStep::setConfigurationMap( configurationMap ); // call parent implementation last
+    setContextProperty( "Welcome", m_config );
 }
 
 Calamares::RequirementsList