LimeReport not really a reusable library

[pasnox]

LimeReport is great, but the LimeReport::ReportEngine is a bit too monolithic to be reused efficiently.

In fact the PrintPreview and Designer windows are not really useful because we can not instantiate them ourselves as it's tied to the ReportEnginePrivate thing.

Please allow those dialogs to be instantiated taking a simple ReportEngine in constructor, this way we can control where and how to instantiate those to suit our needs (modal, non modal, size , screen...).