Add support for configuring fluid threshold conditions by millibucket amount

[ThorGuy]

• When configuring a threshold switch facing into a fluid container, there is now an option to switch between millibuckets (mB) and buckets
  • This means for fluid containers, the menu now uses the same texture when configuring thresholds for item counts.
  • When using a mB value, will increase/decrease by increments of 25mB, or 100mB if shift is held.
    • I am not sure if these step sizes are the most-appropriate.
• When configuring a fluid cargo condition for a train schedule, there is now an option to switch between millibuckets (mB) and buckets.

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Screenshots:

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TODO:

• Language entries for new strings
  • I manually added 'en/us' entries for the above screenshots, but I'm not sure what the proper workflow is for translations so I decided against including them in this PR.
• Texture for configuration menu?
  • Currently, both item and fluid threshold configuring uses AllGuiTextures.THRESHOLD_SWITCH_ITEMCOUNT_INPUTS. At minimum the constant probably warrants renaming since it is no longer being used for exclusively item counts, but I didn't want to go ahead with that change in case it was decided a different menu texture should be used for fluid amounts.
• Backporting?
  • I could not find a contribution guide, and I am very new to contributing to open source, so if this feature were to be backported I would appreciate any help or guidance in accomplishing that.