Wishlist for generators and effects

[Co-eus]

• [ ] g_central_glow (didn't we had this one before?! I checked, it's just missing as an option in gui_2.py, maybe it was not working properly?)
• [ ] g_shooting_star: add option for size change (shrinking, growing) while flying through the cube
• [ ] e_quadrant_cut: "cut" quadrants out (set brightness to 0), could be nice with g_central_glow or any other more blurred generators
• [x] g_cube_edges: turn on random edges of a cube (could mix nicely with g_cube) or let a snake run along the cube edges
• [x] maybe introduce a g_rain again?
• [ ] global effects for channel mixing options: Not only sum the channels (Mix=A+B), maybe also remove B from A (Mix=A-B), since they have different colors, channel B changes the color of A, not only setting it to 0
• [ ] e_gradient or similar
• [x] something with circles

[Co-eus]

• [x] edge_grow or similar - at the corners, lights turn on, travel along an edge and vanish again
• [ ] edge traveller - lights which go along the edges back and forth, but all with a different offset
• [ ] glowsticks - a line turns on (but dark), and a bright sphere 'falls' down
• [ ] growing_cube - a cube which is slowly growing, just be moving randomly leds to the next bigger-sized cube
• [ ] flickering parts of the cube wall, maybe together with colored surrounding lines
• [ ] orbiter with random as mask
• [ ] automat which launches several cube-edges at once or in a short series