Open launch files in window tabs

[yusufm]

Summary

• open pending launch files sequentially so their windows can be grouped
• add additional files from a multi-file open as native macOS tabs
• keep single-file launch opens on the normal document path

Tests

• xcodebuild build -workspace "MacDown 3000.xcworkspace" -scheme MacDown -destination 'platform=macOS'

Related to #408

[schuyler]

Thanks for the PR, @yusufm!

Issues

• Nil document on open failure: when a document fails to open, the nil result flows into MPWindowForDocument and silently breaks tab grouping for every later file in the batch.

Suggestions

• Global tabbing flag: setAllowsAutomaticWindowTabbing:YES runs unconditionally on every activation and persists for the whole session, so scoping it to the multi-file case would avoid overriding the user's system setting.
• Pending list cleared early: filesToOpen is cleared before any open completes, so a failure or crash mid-open would lose the remaining files.
• Brief comments: a sentence on the recursive chain and the "nil tabbedToWindow means this file becomes the anchor" invariant would aid future readers.