Add color-ranges to customizable colors

[westram]

Specification:

• Amount of allocated colors (including start/end color)
  (hardcoded due to ARBs GC-layout)
• Amount of support-colors:
  • 2 (or more) for linear ranges (e.g. black → white)
  • 3 (or more) for cyclic ranges (e.g. red → green → blue → red)
  • 3 (or more) for planar (2-dim) ranges (base-color + one color/dim)
  • 4 (or more) for spatial (3-dim) ranges (base-color + one color/dim)
• Whether to add the first interpolation color to the end of the range
  (e.g. to provide full color-circle: red → green → blue → red)
  provided by separate range-types: linear vs cyclic

Display:

• color selection buttons for support-colors
• a small canvas displaying the color range
  no. Instead allows to show color-range as overlay of target-canvas.

Needed by #443
Has performance issue: #704
See also #672

[westram]

implemented in branch 'color' @ [15005]