Handle transition creation in GScene (instead of GState).
Add transition preview.

* yavgui/src/Makefile.am: Add new files.
* yavgui/src/g_scene.cc,
* yavgui/src/g_scene.hh: Add implementation.
* yavgui/src/g_state.cc,
* yavgui/src/g_state.hh,
* yavgui/src/g_state.hxx: Remove obsolete code, fix (de)selection.
* yavgui/src/g_transition.cc,
* yavgui/src/g_transition.hh,
* yavgui/src/g_vcsn_handler_internal.hxx: Fix item (de)selection.
* yavgui/src/g_transition_draft.cc,
* yavgui/src/g_transition_draft.hh: New.  Floating preview.