BlackBerry: Refactored BarDescriptorDocument

The BarDescriptorDocument now operates directly on the underlying
QDomDocument, and is much better at keeping the tags on their original
lines. Any new values are appended to the end of the document.

A generic API is provided for changing the values inside the document,
BarDescriptorDocument::setValue(..).

BarDescriptorDocument no longer depends on the BarDescriptorEditorWidget, which
should make it easier to implement splitting of the editor.

Task-number: QTCREATORBUG-11012

Change-Id: Icfd681e3af016ea819b99b8cad2cede46600e910
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>
Reviewed-by: Mehdi Fekari <mfekari@blackberry.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>