macOS but generally applicable: reduce redraws on statusbar dramatically improving performance when hovering with paint tool

This change reduces redraws due to spurious status updates when hovering with the paint tool.

Before this change, the status would be unset, then set again to the same value it was previously. Each turn around setting and unsetting causes a redraw.

On macOS this redraw causes a full screen redraw. With this optimization, when the status message is unchanged, which is most of the time, no redraw occurs.

However, due to the coordinates status panel, this change only helps when the coordinates panel is removed or improved. Regardless, this will improve drawing performance somewhat.

This is valid on all platforms even though focused on macOS.

Supports #7690 (closed).