option to display graph times relative to the start time

option to display graph times relative to the start time

This avoids having humongous numbers displayed on the graphs' x
axis. Also, the dialog for adding a data set to a graph was cleaned
up.

* grapher/graph-dialog.glade: Add check box for display of relative
  time and a label for the data set list. Force the list of data sets
  to be larger.
* grapher/Graph.hxx (_timeBase): new member
* grapher/Graph.cxx (draw): Subtract _timeBase from displayed time
  value.
* grapher/GraphWidget.hxx (DataModelColumns): Add a column for a data
  set's title, which is optional at the moment.
* grapher/GraphWidget.hxx (_globalTimeBase, _timeBaseInitialized,
  _relativeTimesButton, _displayRelativeTimes): new members
* grapher/GraphWidget.hxx (GraphWidget): Hook up check button for
  displaying relative time.
  (on_expose_event): Determine base time if needed; set base time in
  graphs.
  (onDataDialogOpen): Insert graph data set's name (key) and title
  into the list of data sets.