QmlDesigner: Refactor component handling

Change-Id: Iba591f255f3d9504b4613e5f712bb94b5ce7c19a
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
QmlDesigner: Add Loader source property handling

You can know go into a component.

Change-Id: I5120e70164a112b277981fda7e603c3d327f2561
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
QmlDesigner: Cleanup openFileComponent

Change-Id: I740bed6dd5b7069c0ebae37a88546bd9d8e94ccc
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>