Add color picker for text color in tab-specific properties

[RealRaven2000]

Advanced tab properties should get a color picker to set the text color:

I think this was working in Thunderbird 60, but didn't work from Tb68 forward as the colorpicker element was decommissioned by Mozilla.

[RealRaven2000]

Implemented in 5.4 - Released 18/03/2021