I finally pushed the changes to the gravity system into the repo. It's a bit different and maybe harder to grasp, therefore I also updated the default config and added lots of comments and information into it. The main advantage of the new system is that gravities now can be fully customized in a suitable manner.
Please have a look at the new default config in /etc/xdg/subtle and also be aware that the :size property of tags is now :geometry. Subtler and subtlext have also been updated for the new system.