tabColor: support options other than RGB

[thomascharbonnel]

Description

Related to #785. I've noticed the tab color feature only supports RGB (read and write) currently. Do you think it would be okay to add support for the other types of tab colors (eg. "theme", "indexed")? I should be able to pick up this task (and the other one we discussed earlier) in the coming few weeks.

[xuri]

Thanks for your issue. Contributions are welcome. I think we need to make following works for this feature:

• Add new options TabColorIndexed, TabColorTheme, TabColorTint for the SheetPrOption
• Implement setter and getter for these options
• Add unit test for this feature

[thomascharbonnel]

Thanks for the help!
I've written some code to support TabColorTheme first, does this change make sense? thomascharbonnel@2e9ea92
I'm not sure what to do when theme is unset, as Golang's default int value of 0 is a valid theme value. I've chosen -1 to indicate that theme is unset. What do you think?

[xuri]

Okay, I agree with you.

[thomascharbonnel]

Thank you for the feedback, i'll keep working on this next week.
I think i'll make the theme an enum (list of possible values: https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.drawing.colorscheme?view=openxml-2.8.1), this way -1 will be a little bit cleaner rather than a magic number.
One strange thing is that Excel does not seem to follow the spec: https://stackoverflow.com/a/2761369

[xuri]

Okay, make the TabColorTheme accept theme enumeration, and returns the theme index as is. In Excel the first four theme color index was different with spec. We need to make a patch for it when getting the theme-based color. For example: get the cell background color.