N.B.: Some changes from plan on ticket
New methods:
Item.setAbstract(true|false) -- make a note an abstract (and clear existing abstract if there is one for source item) or clear abstract status
Item.isAbstract() -- returns true if note is an abstract, false if not
Item.getAbstract() - get itemID of child abstract note or false if none
ZoteroPane.toggleAbstractForSelectedItem()
Changed methods:
Item.updateNoteCache(text, isAbstract)
Notes.add(note, sourceItemID, isAbstract)
Item.setSource() -- moving abstract note to another source with an existing abstract or setting as an independent note will make note not abstract
Other changes:
- Context menu options in items pane: "Set note as abstract" and "Unset note as abstract"
- Child notes are now displayed before child attachments so that abstract will be first
5876debd36