[bugfix] Make PanelDockWidget disable associated display only when pressing close button, not on any visibility change. Fixes #520
renamed PanelDockWidget::closed() to closeButtonClicked(), improved header doc
PanelDockWidget::closed() triggered on any close event, be it the close button or through the toggle action (from the papnels menu). Also, re-enable associated display when visibility set to true from somewhere
Display: made associatedPanelVisibilityChange(),disable() private
Merge pull request #611 from ros-visualization/assoc_widget_fix

[bugfix] Make PanelDockWidget disable associated display only when press...