Gradual select By Angle for polygons

[maddin]

In order to clean up a dense reconstruction, a 'Gradual Select > By Angle' would be great.

So we would select a bunch of polygons, then run this new tool and get a slider to set a crease angle.
The polygon selection would be grown until the algorithm encounters a polygon whose angle to the already selected polygons is greater than the selected crease angle.

This tool should also run in 'deselect' mode, such that we can region-select a larger group of polygons, and then use the tool to deselect planar parts. All the non-planar parts could come from noisy or interpolated data, and this tool would allow for easy selection and deletion of such parts.