Change message display to use hovering window #241
Description
Similar to the tooltips that use a temporary Toplevel window for display (see issue #226), the text shown using the setMessage() method of VisualizationApp should use a hovering window at the bottom of the application window. When the message is cleared, the hovering window should disappear. By avoiding the resizing of the message label widget under the Operations control panel, there should be fewer screen layout changes to distract users.
The message should be centered on the application window with its bottom edge flush with that of the application window. Putting it slightly outside the application window would cause problems when run on the trinket.io site. Messages can contain newlines, making the label widget taller with multiple lines of text. The positioning of the floating message window will need to set to make sure all the lines are visible.
When the application window is moved, resized, iconified, or deiconified (when run on a workstation; not on trinket.io), the message window needs to be re-positioned. This follows the behavior of the adjacency matrix display of the Graph widget.