Add parameters, controlling the behavior and appearance of sliders,
to GimpControllerSlider. The macro GIMP_CONTROLLER_SLIDER_DEFAULT
expands to a nonmodifiable lvalue of type GimpControllerSlider,
whose members are initialized with the most common default values.
Handle the new parameters in GimpToolLine. A slider using the new
"autohide" mode is only visible when selected, or when the cursor
is close enough to the line, between the slider's min and max
values, and no other handle is grabbed or hovered-over.
... so that when the base and balance sliders overlap, the base
slider is the one that's picked, since the balance slider is
constrained by the base, but not the other way around.
Switch the gegl:spiral prop gui from using a line controller to
a slider-line controller, and use sliders to control the "balance"
and "base" properties.
