Move LeafChunk/BranchChunk back to non-class constructors
Move LeafChunk/BranchChunk back to non-class constructors

So that Doc can do its auto-new trick before calling the
superclass constructor.

Closes #4768

wingify/CodeMirror,MarcelGerber/CodeMirror,dperetti/CodeMirror,MarcelGerber/CodeMirror,ficristo/CodeMirror,SidPresse/CodeMirror,hackmdio/CodeMirror,mtaran-google/CodeMirror,SidPresse/CodeMirror,nightwing/CodeMirror,MarcelGerber/CodeMirror,ficristo/CodeMirror,jayaprabhakar/CodeMirror,ficristo/CodeMirror,hackmdio/CodeMirror,thomasjm/CodeMirror,codio/CodeMirror,notepadqq/CodeMirror,pabloferz/CodeMirror,wingify/CodeMirror,nightwing/CodeMirror,grzegorzmazur/CodeMirror,jkaplon/CodeMirror,jayaprabhakar/CodeMirror,thomasjm/CodeMirror,namin/CodeMirror,SidPresse/CodeMirror,grzegorzmazur/CodeMirror,notepadqq/CodeMirror,pabloferz/CodeMirror,dperetti/CodeMirror,jayaprabhakar/CodeMirror,wingify/CodeMirror,nightwing/CodeMirror,hackmdio/CodeMirror,jkaplon/CodeMirror,thomasjm/CodeMirror,namin/CodeMirror,pabloferz/CodeMirror,namin/CodeMirror,jkaplon/CodeMirror,codio/CodeMirror,dperetti/CodeMirror,mtaran-google/CodeMirror,mtaran-google/CodeMirror,codio/CodeMirror,grzegorzmazur/CodeMirror,notepadqq/CodeMirror