Allow processing multiple projects in desktop tool

[ndepaola]

Description

• Resolves Feature Request: Orders over 612 and/or multiple xml files at once #65
• This is an enhanced iteration of the functionality added in Process multiple XML files in batch #103 - I decided to take this in a bit of a different direction to that PR's implementation (though I looked to it for inspiration).
• Key functionality:
  • The XML file select now allows selecting multiple files rather than one
  • XML files can be arbitrarily large
    • There will probably be a frontend PR which removes the 612 card limit - problem for another day
      • The blocker here is slow performance with massive projects
  • Selected XML files are aggregated and then split back out
    • We identify projects which can be aggregated by their specified stock & finish settings
    • For example, consider a 400 card nonfoil S30 XML and a 300 card nonfoil S30 XML - the tool would aggregate these into a single 700 card nonfoil S30 project
    • The tool will then split these projects back up such that each of them fit with the max project size of 612 cards, in one of two ways:
      • Naively splitting every 612 cards, or
      • Asking the user to type in their desired splits
        • This captures the case where the user happens to know it's cheaper to split up the cards differently to the naive approach
  • The tool will then run in a loop on each aggregated-then-split-back-out project
    • The user specifies whether to continue editing an existing order or create a new one per-aggregated project

Checklist

• I have installed pre-commit and installed the hooks with pre-commit install before creating any commits.
• I have updated any related tests for code I modified or added new tests where appropriate.
• Thorough manual testing
• I have updated any relevant documentation or created new documentation where appropriate.
  • On the todo list - unfortunately, github wikis are (kind of?) sub-repositories so I can't update the docs in this PR