Moved text control subclassing code inside MSWOnNotify.

Now all edit controls for tree labels are subclassed (previously only the ones
  created by EditLabel were ).
Now GetEditControl() always returns a pointer to a textctrl when editing
  is in progress.
Removed #if-0 code in GetEditControl() to do the above.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@15172 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775