Doc: filtering BlackBerry 10 messages in Issues pane

Errors and warnings from the BAR descriptor editor are
listed in the Issues pane.

Change-Id: I8ef7c330c00bb6d1c5888f6a06d96b450bc80c02
Reviewed-by: Mehdi Fekari <mfekari@blackberry.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>