Add latte color tag and simplify

[jeremypw]

Fixes #2814

In order to add a color while not affect users' previously assigned color tags and keeping the context menu widget colors in a sensible order it was necessary to distinguish the button index from the color index.

• Add a index member to color button pointing to the required color in COLOR_TAGS
• Add a latte color to the end of COLOR_TAGS and document origin
• Simplify identifying which button was pressed and remove magic constants
• SImplify handling RTL

The simplification could be done in a separate PR if adding another color is not approved.

[teamcons]

Pantheon uses the palette of colors in a few places, and granite apps too.
I did solve it in granite apps by making them an enum, maybe we could granite it up?

[teamcons]

(Thank you <3 )

[jeremypw]

Pantheon uses the palette of colors in a few places, and granite apps too.
I did solve it in granite apps by making them an enum, maybe we could granite it up?

The recommended way is to use CSS with the elementary stylesheet but that does not work with cell renderers unfortunately. Once we have ported to Gtk4 we should be able to use dynamic views with the file items being widgets which we can then style without having to hardcode the color values.