Pipeline: Use preferred size for min and max for sources window.

[PeterJohnson]

Without this, the sources window shrunk when enough processing steps were
added, making it at first hard to read and then impossible to use.

[codecov-io]

Current coverage is 53.69%

Merging #571 into master will increase coverage by +0.02% as of 58c3000

@@            master    #571   diff @@
======================================
  Files          158     158       
  Stmts         4699    4699       
  Branches       500     500       
  Methods          0       0       
======================================
+ Hit           2522    2523     +1
- Partial        140     142     +2
+ Missed        2037    2034     -3

Review entire Coverage Diff as of 58c3000

---

Uncovered Suggestions

1. +0.44% via .../ExceptionAlert.java#83...103
2. +0.32% via ...cketPreviewView.java#88...102
3. +0.32% via ...indowController.java#222...236
4. See 7 more...

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[JLLeitschuh]

Thanks, I've been meaning to fix this for a while now.