Slider/text UC: improved "notch" behavior, with support for custom notching

Slider/text UCs can now individually specify how they want their "notch"
drawn (including an override to not draw it at all).

To keep sliders as clean as possible, a set of heuristics typically
determine whether a notch would be appropriate.  For example, if a
control's value includes 0 as a selectable value, it's assumed that 0 is
the null value for the control.  If 0 lies inclusively within the
control's outer bounds (e.g. it is *not* the control's minimum value), a
notch will automatically be added to the 0 position to help orient the
user.

Some controls do not have obvious "default" values, however.  Opacity,
for example, should default to 100 instead of 0 - and as of this commit,
a custom default value like that can now be conveyed.  Similarly for
things like lens distortion, which is typically applied to the [0.5,0.5]
position in the image, not the [0, 0] position.

Anyway, custom values have been specified in dialogs where appropriate,
which is why this commit is so large...