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

Outline mode can be set up on any layer.

Ratsnest is updated after exporting changes from the module editor to the layout editor.

Introduced SELECTION_CONDITIONS to determine which menu entries should be visible in the SELECTION_TOOL context menu, depending on the selection.