We're finally doing the thing. 


TODO:
- [x] Choose an approach 
- Decided to add a new element just for the pane axis, containing a
slimmed down copy of the flex code.
- [x] Wire through callbacks and pointers so that data goes where it
needs to
- [x] Do the flex juggling math on resize
- [x] Update the flexes when updating the split tree
- [x] Restore the active_pane_magnification setting
- [x] Serialize an axis' flexes

Release Notes:
- Made the center pane group splits resizable. Note that resizing is
disabled if the `active_pane_magnification` setting is changed from
default.