Set the toolbar button's help text. This text is automatically shown in the status bar of
the window that hosts the toolbar whenever the mouse is moved over the toolbar button. This is
quite convenient for the user because it explains the function of the individual toolbar
buttons in an elegant way. See Statusbar class for details.
Note that this is not the same as Toolbarbutton.Tooltip. Tooltips are shown as
little windows right next to the toolbar button if the mouse has been hovering over it
for some time. The help text, on the other hand, is shown immediately in the window's
status bar as soon as the mouse cursor is over a toolbar button.