Add of an empty Datastore tab in Builder

(references #81)
Add of an AdvancedDatastoreDescriptor

(references #81)
Datastore tab now can display, remove or move items

(references #81)
Creation of the Datastore "add item" dialog box structure

(references #81)
"Add item" dialog box implementation for single files

(references #81)
"Add item" dialog box implementation for directories

(references #81)
Add associated files to the "Add item" dialog box

(references #81)
Improve design of the "Add item" dialog box

(references #81)
Reset fields at "Add item" box opening

(references #81)
Update associated files on file path manual change

(closes #81)