[#342, #2538] Display race on sheet, add singleton logic to race & background items

[arbron]

Displays race item above background on features tab of character sheet.

Adds a LocalDocumentField to reference a single item on an actor, and replace background & race properties with this new field. This field has a fallback option so that if the item isn't found, then string is displayed (allowing existing hand-typed Race and Background values to continue displaying properly).

Used a mixin for Item5e and Actor5e that forwards socket event handler requests to their data models, allowing each type to add custom socket event handler behavior. This behavior is used in RaceData and BackgroundData to set system.details.race and system.details.background to reference any newly added items, and unset it when they are deleted. This is also used for any item marked with the singleton metadata property to only allow one to be created at a time.

I've also tweaked all of our current socket event handlers to ensure they return false if the parent handler returns false so we can properly prevent items from being created.

[arbron]

@Fyorl I added a bit of data preparation code to ensure that background labels from before the reference was added continue working displaying properly.

[Fyorl]

LGTM, thank you Jeff