Add icons for gPlanarity

[sled45]

These icons should work. The .desktop file for gPlanarity is hardcoded though, so I opened an issue in hardcode-fixer.

[SmartFinn]

Thanks for the contribution.

The icons have some issues:

1. The icons have poor pixel alignment. The edges of objects do not fit the pixel grid. See Pixel alignment.
2. Outlines, shadows, and the color of the circle do not match Papirus style. See _TEMPLATE@NNxNN.SVG files as an example.
   1. Outlines and shadows should be 1px for 32-64px icons, and 0.5 px for 22-24px icons.
   2. The shadows should be below the objects.
   3. The lighter color of the base circle should be #e4e4e4.

[sled45]

I made changes based off the guidelines and your comments. Hopefully it looks better.