G-code exporter should be split out G-code writing

[Traumflug]

With doubling the feature set over the last year, the G-code exporter's main file, "src/hid/gcode/gcode.c" has become overloaded. It'd be a good idea to split out all the G-code writing into a separate file.

Some 50% of the current exporter handles the distinction between metric/imperial and advanced/basic features. The exporter could hand this over to gcode_writer.c, so it can be stored there and has to be set only once. Then, all geometric stuff could be kept in metric, gcode_writer.c would handle the conversion to inches as required. Possible API:

gcode_writer_fopen()
gcode_writer_fclose()
gcode_writer_set_metric()
gcode_writer_set_inches()
gcode_writer_set_advanced(bool advanced)
gcode_writer_G0()
gcode_writer_isoplunge()
gcode_writer_set_feedrate()
gcode_writer_G1()
gcode_writer_drill()
gcode_writer_comment()
etc ...