Add a full colour gamut picker for text span colour

[ftessier]

The ability to select colour for spans of text inside the text blocks is very useful! It would be even better though if the text block editor colour picker included a full rgb colour picker, similar to the one for objects.

If that is not practical to implement, I would suggest that the default colour swatches for text spans match the ones for objects!

(I understand that I can simply edit <span> colour inside the slide's html, but this becomes tedious!)