Sixth and final Phase 2 site migration. The bookmarks context-menu
Save As action now mirrors main_window._save_as: render the template
to populate the dialog default name, then route the actual save
through save_post_file with explicit_name set to whatever the user
typed. Same behavior change as the browse-side Save As — Save As
into saved_dir() now registers library_meta where v0.2.3 didn't.
After this commit the eight save sites in main_window.py and
bookmarks.py all share one implementation. The net diff of Phase 1 +
Phase 2 (excluding the Phase 0 scaffolding) is a deletion in
main_window.py + bookmarks.py even after adding library_save.py,
which is the test for whether the refactor was the right call.