#9 Improved color table exporting

[Tito Dal Canton]

Unified menu item "Save color table..." for exporting color tables. This item should present a "Save color table" dialog with:
- A "Strip reserved colors" checkbox, which prevents the first 3 colors from being exported (they are reserved colors and one may want to exclude them).
- Two radio buttons for the color table format: "The Gimp" and "PhotoShop".