Read/write heal locations using new JSON data

[GriffinRichards]

• Supports the new JSON format of heal locations as of Generate heal location constants automatically pret/pokefirered#685
• Heal location events can now be added and deleted in the same ways as other events.
• A new warning was added when deleting an event if it has a data field that could also delete a #define from the user's project (heal locations, and soon some object events). The warning can be disabled under Preferences.
• Replaced any mention of "healspot" with "heal location". The projects only use the latter name.

Heal Locations have a respawn_npc field that uses names from an object event's local_id field. Once support for local_id is added the respawn_npc dropdown should be populated with IDs from the respawn_map, and double-clicking a heal location event should select the appropriate event (defaults to 0 right now).