pdInput: fix hand cursor mistakenly extending to non-control picture boxes
Slider/Text UC: drop scroll bar, implement owner-drawn slider

This commit primarily exists to re-establish basic functionality.  I
have a lot more fun stuff planned for sliders, as they're such an
essential part of PD's interface, so this is just the start!

For fun, note that the new owner-drawn slider supports sub-pixel
accuracy.  A good dialog to test this is Adjustments > Lighting >
Shadows and Highlights.  Click one of the sliders to set focus, then
hold an arrow key to nudge the setting incrementally.  Pretty sweet.  :)

As inferred from above, arrow keys are already supported via pdInput.  I
haven't made up my mind yet on whether the sliders should get dedicated
left/right buttons similar to the old scroll bar layout, or whether I
should go with an up/down control for the text box instead.  There are
trade-offs to both approaches.

AFAIK, sliders should be working just fine despite this rather large
change, but the code needs some clean-up and I want to implement a few
other features before syncing the new UI.
Slider/Text UC: code clean-up

- Cache relevant slider values, instead of constantly re-calculating
them on-the-fly
- Remove all old scroll bar code
- Adjust drawing to account for .Enabled property
- Comment apocryphal rendering code
Slider/Text UC: add custom track style property, implement two of them

Custom track styles are going to help PD's many dialogs be much more
user-friendly.  I've only implemented two styles so far; the others are
forthcoming:

1) Default style.  The track bar will be filled between it's "minimum
change" position (0 if 0 is supported as a value, or the control
minimum) and the current slider position.  If the "minimum change"
position is *not* the control minimum, notches will be displayed to help
orient the user.

2) "No-frills" style.  I don't know if this will ever be necessary, but
it's there; basically the track is never filled, regardless of slider
position.
Add x/y slider preview updates to Lens Flare and Sunshine dialogs

My bad for missing these before!
Slider/Text UC: add Gradient track modes (both 2 and 3 point)

Two-point gradients are used when the background track of a slider
extends smoothly from one color on the left to a different color on the
right.

Three-point gradients are similar, except they allow for a custom middle
color (and corresponding middle value) somewhere between the left and
right color.  This is presently demonstrated in two places: Adjustments
-> Color Balance, where Gray is used as a midpoint on each color slider.
And Adjustments -> Lighting -> Temperature, where a non-center midpoint
is used to demonstrate 6500 K.

Similar to the default track mode, a "notch" is automatically added at
the middle location of three-point gradient tracks.  (Again, see the
Color Balance dialog for a demonstration of this.)
Color Balance, Temperature Adjustment dialogs: add gradient colors to relevant sliders

I love that the slider controls live-update while in the IDE.  :)

Lots more dialogs need this feature manually added, but these two
provided a good test-bed - color balance for demonstrating a standard
use of the three-point gradient feature, and color temp for making sure
modifying the gradient colors at run-time doesn't cause any trouble.  So
far, so good!