ModEdit: fix some issues due to the fact legacy footprint libraries cannot handle 32 copper layers, and are therefore no more editable:
* in file/library save as..., remove option to save a lib under the legacy format (which is not possible and creates an error message)
* if the current loaded lib is a legacy lib, when the user try to save or delete a footprint, displays a message which explains what the user should do.
* add a suitable dialog to select/create a .pretty folder library (a file dialog or a dir dialog coming from wxWidgets is not friendly usable.)

3d viewer: Add high quality mode option is realistic mode (Shows holes in copper zones, but with longer calculation time)
Gal: fix a very minor issue: the keys to switch between copper layers are now - and + (according to the doc and the normall mode), instead of - and =

Add interactive router doc, from Tomasz Wlostowski (see github.com/twlostow/kicad-dev/blob/junk/Documentation/interactive_router)
Remove the outdated pns_router.txt file.

fix Bug #1345193:  Drill Map coordinates alway 0 when using Gerber format (due to a missing plotter initialization).

Fix crash in Pcbnew and Gerbview when selecting (in layers manager) "always hide all layers but active" from popup menu

Fix crash in Pcbnew and Gerbview when seceting (in layers manager) "always hide all layers but activer"

Apply a workaround to fix a wxWidgets  bug in versions 3.0.x relative to mouse wheel event (Windows specific).
Therefore wxWidgets 3.0.1 can be used without need to patch it.
(This bug is fixed in wxWidgets 3.1)

Better separation between Pcbnew code and Gerber code.
Update export to Pcb code (uses now the version 4  kicad_pcb file format).

Gerbview: allows x.7 format (recently indroduced in Gerber format)
Also minor coding style fixes.