Remember layout across restarts (PR for client properties)

This a proof of concept to remember layout properties across restarts.

For now, only the floating state of clients is remembered.

This is meant to collect feedback and then also apply it e.g. to the selected layout of tags.
What else should be covered and restored?

Ref: https://awesome.naquadah.org/bugs/index.php?do=details&task_id=759