Allow Arc source to be downloaded externally

[ReillyBrogan]

• Adds the --arc-directory parameter that allows the arc source to be downloaded externally (this is mostly for use by packaging systems that prefer that they manage the source, usually for caching and reproducibility purposes)
• Changes $ARCVERSION to $ARC_VERSION to match how the other variables are named.
• Allow $ARC_VERSION to be specified via the --arc-version flag
• Update the validated Arc version to 20210412
• Have the script copy from the pre-downloaded arc source or the downloaded arc source into a separate arc-theme working directory instead.
• Update the inkscape command to work with the newer actions syntax. The old --export-plain-svg flag is deprecated and generates a lot of warnings when the script is ran
• My previous MR should have ran sed against meson.build instead of meson.options. This has been corrected and the output of that part of the script has been clarified.
• Replace the find -exec logic with find -print0|xargs so that we can multithread that file processing. This dramatically speeds up the build on my machine. The worker threads default to the number of CPUs, but this can be overridden with the --jobs flag.

[ReillyBrogan]

On my system (which is a Ryzen 5900x on a decent NVME SSD) I timed the script pre-xargs:

Executed in  311.58 secs    fish           external
   usr time  263.13 secs  461.00 micros  263.13 secs
   sys time   64.86 secs    0.00 micros   64.86 secs

And post-xargs

Executed in   32.71 secs    fish           external
   usr time  337.08 secs  235.00 micros  337.08 secs
   sys time   72.87 secs  124.00 micros   72.87 secs

Both of these were executed with a pre-downloaded Arc source to try to minimize that from being a factor.