Modular-Kicad milestone B), major portions:

*) Rework the set language support, simplify it by using KIWAY.  Now any major
   frame with a "change language" menu can change the language for all KIWAY_PLAYERs
   in the whole KIWAY.  Multiple KIWAYs are not supported yet.

*) Simplify "modal wxFrame" support, and add that support exclusively to
   KIWAY_PLAYER where it is inherited by all derivatives.  The function
   KIWAY_PLAYER::ShowModal() is in the vtable and so is cross module capable.

*) Remove the requirements and assumptions that the wxFrame hierarchy always
   had PCB_EDIT_FRAME and SCH_EDIT_FRAME as immediate parents of their viewers
   and editors. This is no longer the case, nor required.

*) Use KIWAY::Player() everywhere to make KIWAY_PLAYERs, this registers the
   KIWAY_PLAYER within the KIWAY and makes it very easy to find an open frame
   quickly.  It also gives control to the KIWAY as to frame hierarchical
   relationships.

*) Change single_top to use the KIWAY for loading a KIFACE and instantiating
   the single KIWAY_PLAYER, see bullet immediately above.

*) Add KIWAY::OnKiwayEnd() and call it from PGM_BASE at program termination, this
   gives the KIFACEs a chance to save their final configuration dope to disk.

*) Add dedicated FRAME_T's for the modal frames, so m_Ident can be tested and
   these modal frames are distinctly different than their non-modal equivalents.
   KIWAY_PLAYER::IsModal() is !not! a valid test during the wxFrame's constructor,
   so this is another important reason for having a dedicated FRAME_T for each
   modal wxFrame.

On balance, more lines were deleted than were added to achieve all this.