macos: reduce frequency of coordinates refresh thereby dramatically improving performance

This change reduces to 3.33 times a second, the updates to the status bar coordinates widget.

This dramatically improves frame rate on macOS retina displays because it reduces the frequency of full screen updates which are triggered by this widget updating and are very slow.

This is one solution to the slow display problems, but a better solution might be to move the box to the top of the canvas. Or find a more general solution in the Gtk internals.

Supports Issue #7690 (closed).

This MR is now Mac specific in order not to hurt other platforms, but it has a dramatic impact on paint tool performance on a macOS retina display.

The vast majority of Macs now have retina displays, with Macbook Pros switching over in 2016 and Macbook Airs in 2018.