Universal Drag/Drop Reordering of Sections Between All Views

At present, the three LyX-Outline Views (Corkboard, Expanded Outliner, Document Map), all implement drag and drop reordering of sections in a different way using five different LyX-functions. The drag/drop should be combined between all views that share a common model.

Development Tasks:

1.) Implement a single LFUN capable of both moving and changing the indentation level of a particular item. (Done)
2.) Implement DataManager functions that make drag/drop reordering easier and more consistent. (Done)

3.) Create a subclass of QTreeView that use the improved drag/drop methods and can serve as the basis for TocUI->tocTV and the expanded Outliner view.