G/R/S visualization overlays

[Keavon]

G/R/S, used with both the Select and Path tools, should display an X or Y line when grabbing or scaling in the X or Y axis. (The line should be aligned with the common center, i.e. the pivot used for scaling or rotating.) It should go from one end of the viewport to the other. It should also display a dot around the pivot while rotating or scaling. Reference how other overlays are drawn (like those visualizing snapping relationships) for how to draw overlay shapes like lines and dots.

Also, text should be drawn next to those lines visualizing the numerical offsets. Currently these values are shown in the hints bar, which is a temporary workaround until this is implemented.

Also: a line should be drawn when dragging with the Select tool while holding Shift to constrain it to the X or Y axis. (That behavior already exists, but the line should be added to visualize it.)

• Add visualization overlays to G/R/S #2195
• Polish the G/R/S feature behavior, visualizations, and hints #2229
• Further polishing of G/R/S visualization and features #2243
• The visualization overlays don't correctly handle panning (e.g. with the mouse scroll wheel) or zooming.

[mTvare6]

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