The signal is emitted when a slider is dragged away from the line,
and will be removed when the button is released, and when the
slider is dragged back to the vicinity of the line, and won't be
removed.  The last parameter of the signal is a boolean flag
differentiating between the two cases.

Note that a remove-slider signal may be emitted without a preceeding
prepare-to-remove-slider signal, however, is a prepare-to-remove-
slider signal is emitted with a TRUE last parameter, it must be
eventually followed by a remove-slider signal, or by another
prepare-to-remove-slider signal with a FALSE last parameter.