Listen to sample holder insertion events
gui acquisition: allow snapshot as soon as the tab has some streams

We were too picky, expecting the _focused view_ to have some streams,
even if eventually all the streams would be saved. In addition this caused
problem when loading a new image in the gallery, because this would not
be detected until a new view would be selected.
Merge branch 'master' of vpn.delmic.com:odemis