Improved fix for #3889 (Middle click should close files):
- Don't go through normal selection-click handling codepath first: detect
middle click in the same place we filter out close-button clicks. No longer
need a separate click listener as a result.
- Use constants for event.which
- Remove long-commented-out rename code
- Move misplaced comment ("might never have been opened") to its rightful
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Merge pull request #4576 from adobe/pflynn/middle-close

Improvements to middle-click closing files