Set border color for TextAreas

Please, add a control sequence to set the color of the border in TextAreas, in a similar way as currently the area color is set.

changed milestone to %v15.0.0~alpha.2

added graphics; css; layout label

mentioned in issue #999 (closed)

created merge request !981 (merged) to address this issue

mentioned in merge request !981 (merged)

mentioned in commit 6ce765c9

closed via merge request !981 (merged)

changed milestone to %v15.0.0~alpha.0