Improve positioning of color legend horizontal title when in vertical orientation

When the color legend is in vertical orientation and the title is forced to be horizontal, change the anchoring points for left, center, and right justifications to be relative to the bounding box of the color bar AND the axis (which includes ticks and labels). This improves the title position relative to the rest of the color legend. Note that annotations are not considered when positioning the title.

Examples:

Completes #17570 (closed).