Filament settings

I'm working with different materials in a multi-extruder setup. Thus I need individual filament_diameter, nozzle_temperature etc. per extruder. Although Slic3r internally handles those parameters per extruder, there is no obvious way to configure this.

As a first approach I simply moved the parameters filament_diameter, extrusion_multiplier and extruder temperature from the Filament Settings tab to Printer Settings and added per-extruder information to the Gcode header.

The simple-mode is not affected since it doesn't support multiple extruders.

Feedback, tests and hints appreciated.