RCP: X.X, update start annotation enable state when annotation structure is updated

add a corpus listener to the StartButton and catch StrutureEvents. Update the enable state to true if the annotation structure is not empty and update to false is the annotation structure is empty

(from redmine: issue id 2021, created on 2017/01/30 by Matthieu Decorde)

added annotation type::feature + 1 deleted label

changed milestone to %TXM Annotation URS 2.0

if the corpus is saved, the start/save button is disabled
if the corpus annotation structure is modified and not empty, the start annoation is enable

(from redmine: written on 2017/01/30 by Matthieu Decorde)

closed

assigned to @mdecorde