[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

[taylor.smock]

Initial patch (pre-existing tests pass)