2022-May-02 - Prerelease - 2.6.730

Add an "Image Cleaner" map making tool. (#10401)

* Add an "Image Cleaner" map making tool.

This tool can save effort for map makers by handling map image files that are not as strict as the ones needed by the polygon grabber. It does the following:
1. Normalizes colors by changing pixels that aren't white to black.
2. "Fills in" small regions (below `minimumRegionSize`) with black pixels.
3. Removes "unnecessary" black pixels by turning them to white, making the resulting lines between regions have a thickness of 1 pixel.

The new functionality is provided via new menu items from the polygon grabber.