Add a Theme variable to draw thicker focus rectangles (buttons, etc.)

Noticing on high-resolution screens (such as 4K Retina displays on Mac) that the one-pixel wide focus rectangle lines are not very visible, nor are they when drawn on top of the Command buttons (dark blue backgrounds).

So, changing the width and spacing of the dashed line strokes helps with this.

But, this may not be necessary in some situations, so make a new theme property to either use the old default value or the new "thick" version.