gtkbuilder: add accessibility role declaration

This allows to override the role declared to the atk stack.  For
instance,

<accessibility>
  <role type="static"/>
</accessibility>

allows to tell the accessibility stack that a label is just a message in
a message box.

Fixes #109
1 job for accessible-role in 19 minutes and 5 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build #15780

00:19:05