Context-menu string cleanup

- Remove all "Selected" from "Selected Item", because that's how a context menu works
- Change "Remove Selected Item" to "Remove Item from Collection" (fixes #122)
- Change "Delete Selected Item from Library" to "Move Item to Trash")
- Change "Remove" on Duplicate/Unfiled Items to "Hide" (fixes #122)
- Change "Create Parent Item from Selected Item" to "Create Parent Item"