AGS 4: Editor: Color Picker dialog with alpha selection

[ivan-mogilko]

Since we have alpha component in color properties now, we need a new Color Picker dialog with alpha level selection.
Need to either develop one ourselves, or find an available control elsewhere.

Requirements:

• Allows to select RGB on a color shape by clicking on it (color wheel or rectangle);
• Allows to change Value (if HSV) or Brightness (if HSB) by dragging a slider;
• Allows to change alpha by dragging a slider;
• Allows to input all 4 ARGB values in text fields by hand.
• Ideally, you should be able to copy/paste ARGB value in certain format from elsewhere (in (A,R,G,B) and (0xAARRGGBB) formats).
• Optionally has both RGB and HSV inputs, but latter is not critical.
• Optionally has a "Transparent Color" button for quickly selecting fully transparent color.

As a bonus task (may be split out into a separate ticket):

• Allows to create a custom palette right in the picker dialog for quick color selection, similar to classic color picker in Windows. This palette is serialized as either project data, or workspace data (Game.agf.user file).