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.