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

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.

- zone filling algorithm
- printing
- plotting (pdf & gerbers)
- SVG export
- Specctra export
- 3D viewer