Graphical mode synchronization

Hooks the trace synchronization code in lttvwindow. This allows to use trace
synchronization in graphical mode when lttv is started with the "--sync"
option. Unfortunately, the viewer interface "freezes" while the
synchronization code is running. This can take a noticeable amount of time
(more than a minute) for large traces.

Signed-off-by: Benjamin Poirier <benjamin.poirier@polymtl.ca>