Add TextColour and HasTextColour to Button

Buttons can take a custom background colour (shipping in the next release) and the caption automatically switches to black or white for contrast. Some designs need direct control of the caption colour instead — for example keeping a brand colour on the text, or a translucent button with coloured text on a photo background.

Add TextColour and HasTextColour to Button, matching the existing BackgroundColour/HasBackgroundColour convention:

• Assigning TextColour enables the override and paints the caption (and symbol) in that colour.
• HasTextColour = False restores the default appearance without discarding the stored colour.
• An authored text colour takes precedence over the automatic contrast colour, and keeps its exact value across hover and pressed states on macOS, Windows, and Linux.
• With no text colour and no background colour authored, the button continues to follow the light/dark theme exactly as before.