CreateStandardColorMap Refactor

[tarkpate]

Changes:

• Fixed value scaling for alpha values in ColorMap
  CreateStandardColorMap changes:
• Set UI size to fixed and changed custom color map buttons
• Fixed value range for color map preview so the selection range is always in [0,1]
• Preview points can be reselected and dragged
• Preview points can be deleted with middle mouse button too, so it can be cancelled when you haven't dropped a point yet
• Preview points are clamped to the nearest neighbors, which solves the problem of having multiple points at the same x value

[dcwhite]

Issues to test this with:

• Adjust alpha hues in createstandardcolormap, part 3 #2006
• CreateStandardColorMap improvements, part 2: improve performance of GUI #1349
• SCIRun crashes when changing the colormaps #1127

[tarkpate]

Issues to test this with:

* [ ]  [Adjust alpha hues in createstandardcolormap, part 3 #2006](https://github.com/SCIInstitute/SCIRun/issues/2006)

* [ ]  [CreateStandardColorMap improvements, part 2: improve performance of GUI #1349](https://github.com/SCIInstitute/SCIRun/issues/1349)

* [ ]  [SCIRun crashes when changing the colormaps #1127](https://github.com/SCIInstitute/SCIRun/issues/1127)

Thanks for linking those. We should be able to close #2006 with this. I'll test performance on my old macbook to see if it's still necessary. I didn't get any crashes on linux, so I'll test Mac

[dcwhite]

Layout of the UI needs some fixing on Mac (and likely elsewhere)

[dcwhite]

Top is PR, bottom is original.