Allows forcing the widget to have a specific fixed range width,
so that interactions with the lower or upper slider automatically
force the other slider to move to keep a constant width
In horizontal mode left/right,pageup/pagedown changes active slider position
while up/down flips between selecting the upper, lower or range control
In vertical mode up/down/pageup/pagedown changes the active slider position
while left/right flips between the subcontrols
