New enemy encounter parsers, GUI changes and more

Added EncounterParser and EncounterGroupParser w/ editing capabilities;
changed TableEntryEditor's tree building and node arrangement, added
TreeNodeCategory attribute for that; generalized some things, ex. the
former ItemNameConverter (now DictionaryStringConverter w/ derived
classes); changes to data via parser now update the entry editor's node
names, other changes still need full rebuild; some other less
important/interesting changes