Dashboard: Add stale battery, group labels, fix test

[erelson]

List of changes:

• I add the ability to set a battery to be stale, but this is not used by default. In the Fetch Robotics implementation, I have a timer linked to a rostopic which makes the battery stale under certain conditions.
• Labels for widget groups now optionally implemented by nesting the list of widgets in another list, accompanied by the label (as a string). Then, the groups of widgets are added to a QGroupBox with a QBoxLayout
  • This necessitates invoking setIconSize on the various widgets (with exception handling because the battery widget is using a QLabel instead). (setFixedSize does not work for this).
  • self.setFixedSize(self._icons[0].actualSize(QSize(50, 30)) is currently invoked in most widgets. Seems like it would make sense for this to be done with the dash's max_icon_size at the same time that I invoke setIconSize (so, i.setFixedSize(i._icons[0].actualSize(self.max_icon_size))). However, since this would affect other repos (e.g. rqt_pr2_dash uses this on each of it's re-implemented widgets), changing it here would be bad.
• The sole unit test didn't work because the battery widget's name variable was private. Fixed.

These should be backwards compatible. I've tested with the pr2_dashboard pointed at a Fetch robot with both the current master and this pull request. Visually there are no changes (though I had to explicitly set some stuff on the QGroupBox and QBoxLayout to achieve this). No errors/warnings occur in either the pr2 or fetch dashes, either.

Reference rqt_pr2_dash (aka what I see on my computer when I run rqt and add the rqt_pr2_dash):