[Feature Request] Use current selected palette/resource for color picker instead of the system's native color picker

[Superjustinbros]

SLADE Version: (3.2.5) Operating System: Windows and OSX

Issue Details: For a while, i've been using SLADE's image-editing options for last-minute touchups to graphics, sprites, textures, etc. However, my operating system's native color picker (both Windows' and Mac's) is impractical for picking colors to use in editing graphics, as the colors it gives me are not paletted to match the current 256-color palette I'm editing with. And while you can add custom colors to reference from later, the number of slots for custom colors are limited, and on Windows, any colors you save are erased when you power off the computer.

Perhaps by default, choosing a color to draw with could instead bring up a window that displays and lets one pick from any of the colors in the current selected palette, with a side option to open the default Windows/Mac color picker.