lcm-spy did not display content of some messages

lcm-spy in LCM 1.2.0 does not properly display the content of some messages.
If you start lcm-spy and right click, or double click on a message in lcm-spy:s main panel,
lcm-spy will display a new panel that is supposed to show the contents of the message.

lcm-spy decides which message fields to display, based on the content of ObjectPanel.visibleSparklines.
However, there is a bug that causes visibleSparklines to sometimes not be initialized.

This pull request changes the handling of visibleSparklines so that it is initialized by the paint() method,
if needed.