[PATCH RFC] Allow layers to determine autosave functionality

[taylor.smock]

The patch currently:
1) Modifies ImageData/NoteData to implement Data, but they return an empty collection of DataSources
2) Modifies AbstractModifiableLayer to have a function for autosave (takes a file argument), return false by default (to indicate not saved and not savable). It also adds an abstract method to get the data from the layer. The only addon I found to be affected by this was OpenQA, which can be modified by me.
3) AutosaveTask now just has a single Set<Data> instead of one each for DataSet/NodeData.
4) There is now a global instance of AutosaveTask, once it has been initialized. ~I should probably change all calls to be to getInstance~.
5) It iterates through the layers, using the same if (changedData.remove) syntax, and then calls the layer's autosave method.
6) AutosaveTask also has a new method to indicate that data has been updated
7) Affected layers now implement getData and autosave, where the file was previously autosaved

EDIT: Mapillary is also affected

[taylor.smock]

Initial patch (pre-existing tests pass)

[Don-vip]

Replying to taylor.smock:

4) There is now a global instance of AutosaveTask, once it has been initialized. I should probably change all calls to be to getInstance.

Why? I don't see the need.

[taylor.smock]

I haven't done that yet (in fact, I don't remember why I was thinking I might have to do it). (I'm assuming you are talking about the getInstance I was referring to).