Refactor code to use Argparse instead of GetOpt for improved readability + flexibility

[Iridium-IO]

Argparse

• Commandline parameters now use argparse() instead of getopt(). This reduces the need to provide separate help/default/variable assignments and makes it easier to manage new parameters. This change is feature-equal to the old getopt version, the only real difference being that command-line arguments now always override config-file arguments
  • See: Refactor code to use Argparse instead of GetOpt for improved readability + flexibility #43 (comment)
• Custom argparse definitions and functions have been allocated to gcodeplotutils/argparse_c. This mainly has custom extensions to handle features requires for parity with the old system (comments in config files, --no prefixes, etc)

Inkscape

• GCode can now be exported using File > Export in Inkscape. This allows you to keep a degree of separation between the SVG and exported GCode (plus it's fewer clicks when rapidly testing, as you can keep the Export window open in a tab. Saving normally still works as expected.
• The layout for the GCodePlot dialog has been improved with better grouping of related variables
• Extract only one color from drawing now uses the new Color selector in Inkscape instead of passing RGB values as a string

General

• Simplified __main__ to reduce nesting and make code easier to read
• Added comments to __main__ describing different sections

Note
This PR also includes the changes from #42 adding Moonraker support. Sorry, I didn't quite know how to separate the changes without deleting all the Moonraker stuff, creating this PR, then re-adding the Moonraker stuff. I don't want to clog up your repo with changes and un-changes, but if you're more happy with this PR (as it's more testable since you don't have Moonraker), I can do the following steps:

• Remove Moonraker support from within this branch (IridiumIO/GcodePlot/argparse) which will automatically update this PR
• Merge this PR into your base repository if accepted
• Pull these changes into the Moonraker PR which can then sit open until someone else tests it.

Thoughts? I've tested the generated GCode using my own tests (complex overlapped shapes with variable shading/cutting) as well as running the included mpcnc and cutter config files to check that the output gcode is identical. There is some variation in both the new and old files due to non-deterministic choosing of which shapes to draw first (e.g. one time it will draw star > circle > square then the next time you run the script it will draw circle > star > square

[arpruss]

I would love to merge everything other than the argparse, because the change in config file usage could break somebody's shell script.

[Iridium-IO]

I'll see if I can figure out how to keep it consistent so old scripts work too :)

I probably should've use optparse instead of argparse, but I didn't even know that existed at the time

[Iridium-IO]

@arpruss okay the old way of using config files should work again now, I've tested using the included mpcnc files and they work.

I did make one small change, though: command-line arguments now always override values in the config file, regardless of the order they're provided. Previously, the behavior depended on the order the --config-file argument appeared.

Given a config file:

pen-down-speed=5

Old outcome:

gcodeplot.py --pen-down-speed 10 --config-file cfg
>> pen-down-speed = 5

gcodeplot.py --config-file cfg  --pen-down-speed 10
>> pen-down-speed = 10

New outcome:

gcodeplot.py --pen-down-speed 10 --config-file cfg
>> pen-down-speed = 10

gcodeplot.py --config-file cfg  --pen-down-speed 10
>> pen-down-speed = 10

This probably won't matter for most users (since mixing config files and CLI args probably isn't common), but I think it's more intuitive: if someone explicitly passes an argument on the command line, it should take precedence over the config file.