TextColour cannot be restored to the theme after being set

On Label, Link, TextField, TextArea, SearchField, and NumericTextField, assigning TextColour permanently replaces the themed text colour. There is no way to return to the default: once any value has been assigned, the control stops following the light/dark theme, so an app that adapts its colours loses the correct caption colour in the other appearance. On Link, the assignment also replaces the link blue together with its hover, pressed, and visited state colours.

Fix this with the same convention as Button.BackgroundColour: add HasTextColour to each control. Assigning TextColour enables the override, and HasTextColour = False restores the theme-following default without discarding the stored colour, so the value can be re-enabled later. On Link, restoring returns to the standard link colour and its interaction states.