Issue39v2

[skowront]

As suggested in issue 39, the application now works like any normal hierarchical view. It means what follows:

1. It is possible to select multiple items using Ctrl+LMB (toggles selection) or Shift+LMB (selects range). There was also added a Ctrl+A option (select all when not all selected, and deselect all when all selected).
2. It is possible to drag & drop multiple items.
3. 1&2 applies for groups, so drag&drop from/into different groups is possible.
4. When multiple items are selected some context menu options are disabled - like 'rename'.
5. It is possible to delete multiple selected items both with context menu and with 'Delete' key.
6. Clicking on group automatically selects all items within the group.
   Unfortunately because of built tree view capabilities the solution was achieved by redundant items collection of selected items working in parallel to the GongSolutions drag&drop. Redundant and gong code lines converge in GongSolutions Drop which handles arguments from both sides.