QmlDesigner: DesignDocument removing m_currentModel

We remove m_currentModel and replace it by currentModel().
If m_inFileComponentModel is nil the current model is the document
model, otherwise the current model is the inFileComponentModel.

No need to keep m_currentModel around explicitly.

Change-Id: I590e02dc03c2319cbf713b880cf43d740d64e942
Reviewed-by: Marco Bubke <marco.bubke@digia.com>