Update "Threshold image" tool

[kostrykin]

For the contributor:

• [x] - I have read the CONTRIBUTING.md document.
• [x] - License permits unrestricted use (educational + commercial).
• [x] - This PR adds or updates a tool or tool collection.
  • [ ] - This PR adds a new tool or tool collection.
  • [x] - This PR updates an existing tool or tool collection.
  • [x] - Tools added/updated by this PR comply with the Naming and Annotation Conventions for Tools in the Image Community in Galaxy (or explain why they do not).
• [ ] - This PR does something else (explain below).

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Update "Threshold image" tool:

• Replace dark_bg by invert_output
• Add manual thresholding, might be useful for #105
• Update UI

Update notes: When updating workflows containing this tool from previous version 0.0.5 to now 0.18.1, set the invert_output option if dark_bg was not set, and unset the invert_output otherwise (this is the default).