* Use only double to store zoom levels in class BASE_SCREEN (int type is not working for eeschema or page layout editor) (this fix Bug #1450551, Assertion failed on wxRound() when zooming)
* Fix crash in all apps but Pcbnew when a grid or a zoom was selected from the popup menu.
* Modify default hotkeys for Next Grid and Previous Grid: the initial key was a dead key, not suitable as hotkey. the 'N' and Shift N keys are used instead (with a change for unfill zone hotkey)
* Modify code in EDA_DRAW_FRAME::SetNextGrid() and EDA_DRAW_FRAME::SetPreviousGrid() to make them compatible with other editors than the board editor.
this option was possible only by modifying the way display options are managed.
before these changes, display options were a global DISPLAY_OPTIONS class instance.
Now each EDA_DRAW_FRAME(and derivated classes) includes its own DISPLAY_OPTIONS class instance.
As a consequence, some duplicate display option variables in these classes have been removed, because there were just duplicate variables of the DISPLAY_OPTIONS class instance.
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.