Picom shadows - only on floating windows [feature request]

[Pebor]

Picom shadows can be added easily but there is a catch in that, that it doesn't look good in tiling scenarios. Adding a property to a window to distinguish if its floating or not would open up the possibility of it being filtered by picom when applying shadows. i3 has it's I3_FLOATING_WINDOW property that can be filtered through as a example.

This feature was approved by the developer , feel free to make a PR if you know how to create such a thing.