Customise GroupBox border colour, thickness, corner radius and caption colour

Allow GroupBox appearance to be customised through the Inspector and in code using four properties:

• Border colour.
• Border thickness, with zero hiding the border.
• Corner radius, with zero producing square corners.
• Caption text colour.

These settings will let developers adjust the appearance of grouped controls without creating a replacement control. Preserve the existing appearance when no custom settings are applied, and allow custom colours to be reset to the theme defaults.