DIALOG_EXCHANGE_MODULE class now has its own header file in the
pcbnew/dialogs directory, and the xchgmod.cpp files was moved along with
it. This has two aims:
* (minor) Standardise the location of "base dialog" derived classes,
like most other dialogs in KiCad.
* (major) Allow inclusion of this dialog class into both legacy-mode
pcbframe.cpp and into GAL tools.
John Beard