layerling 1.56.0

• No moiré under a workplane on the plate: A workplane put on the empty plate lies in the plate's own plane, and the two grids, slightly offset, made a flickering moiré. While a workplane lies flat on the plate, the plate now shows only its outline; a tilted or raised workplane keeps the plate's grid. Reported by @prmod3d in #178.
• Corners and edges of the view cube: A click on a corner of the view cube now looks at the model diagonally from that corner, and a click near an edge looks from that edge, between two sides; the spot lights up under the pointer. Dragging the cube and the keys 1-6 work as before. The MCP bridge's layerling_capture_image takes the same views as faces joined with "-", like front-right-top. Suggested by @hlmodtech in #202.
• Gears with involute teeth: New gears have involute teeth that mesh, set like a gear generator: number of teeth and module, which give the size (outside diameter module x (teeth + 2)), plus pressure angle (20° by default) and backlash for printing (0.2 mm per pair by default). Select two gears and the properties panel gives the centre distance they mesh at and how far apart they stand now, or says their modules differ. A helical gear's angle is then the true helix angle, so a pair meshes with the same angle, one positive and one negative; the bevel gear takes the same teeth, tapered, as an approximation. The exact body draws the flanks as smooth curves, so edges can be filleted. Older designs keep their straight teeth, now called "Simple", until the tooth shape is switched; the shape defaults in the settings take the tooth shape too. The MCP bridge takes gearProfile, gearPressureAngle and gearBacklash. Suggested by @prmod3d in #201.