Add supprt for placing sliders on a GimpToolLine -- handles that can
be dragged over the line.  The sliders are accesible through a new
"sliders" property, and via the gimp_tool_line_{get,set}_sliders()
functions.

Add a slider-line controller, which works like a line controller,
but whose callback also supplies/takes an array of sliders.

Note that the data type for individual sliders is called
GimpControllerSlider (in particular, it's not line specific), so
that we may use it with other controller/tool-widget types in the
future.