Split the floating group into off into its own package

This is an attempt to commence organizing StumpWM into smaller parts, as discussed in #125.

I'm not sure if this should be merged as-is, but I'm curious to hear your thoughts. Splitting some parts off into their own package might be a good way to organize things a bit better. Maybe this will also involve finding some higher level abstractions in some places.

I chose to start with the floating group because it's a relatively uninvolved element of the system. Other things, like the tiling group, spread out a lot farther through different parts.