* Create a generic yes/no/cancel dialog from DIALOG_EXIT.
* Make DIALOG_EXIT return wxID_YES instead of wxID_OK so it is consistent
  with the standard message dialogs.
* Add missing license to confirm.h and confirm.cpp.
* Change Eeschema message dialog when loading a schematic if the current
  schematic is modified to be more consistent with the exit dialog.
* Change Pcbnew message dialog when loading a board if the current board
  is modified to be more consistent with the exit dialog.
* Remove some Eeschema block debug logging code left over from my last
  commit.