2) Change from legacy Cu stack to counting down from top=(F_Cu or 0).
   The old Cu stack required knowing the count of Cu layers to make
   sense of the layer number when converting to many exported file types.
   The new Cu stack is more commonly used, although ours still gives
   B_Cu a fixed number.
3) Introduce class LSET and enum LAYER_ID.
4) Change *.kicad_pcb file format version to 4 from 3.
5) Change fixed names Inner1_Cu-Inner14_Cu to In1_Cu-In30_Cu and their
   meanings are typically flipped.
6) Moved the #define LAYER_N_* stuff into legacy_plugin.cpp where they
   can die a quiet death, and switch to enum LAYER_ID symbols throughout.
7) Removed the LEGACY_PLUGIN::Save() and FootprintSave() functions.
   You will need to convert to the format immediately, *.kicad_pcb and
   *.kicad_mod (=pretty) since legacy format was never going to know
   about 32 Cu layers and additional technical layers and the reversed Cu
   stack.

   to a new BOARD in the footprint editor.  This is PAGE_INFO::A4 and company.
   
*) Remove static storage of the BOARD in the module editor.

Change class PROJECT to use a generalized wxString in place of the
RETAINED_PATH items, so that new session and project specific strings
of any purpose can be saved there, for the life of a session.

Page layout editor: fix bad initialization on startup, which can crash pl_editor when trying to edit an item.
Minor other fixes.

Fix segfault when deleting last user path from eeschema libpaths and then pressing cancel.

remove global g_UserLibDirBuffer which was a project specific global, and no longer used with FP_LIB_TABLE support.

.kicad_mod files: always save the "last edited date" because only the footprint editor changes this parameter. Usefull to know if/when a footprint was actually modified.
Also fix a minor issue in footprint editor which always set the "modified" flag even when no change was made.