Refactor: Create doc::Context and move some logic from app::Document to doc::Document

The objective of these changes is to create a more testable doc/logic API.

Included changes:
- Added doc::Context, doc::Documents (the old app::Documents),
  and doc::Sprites (and observers for each)
- Added raster::Sprite::createBasicSprite()
- Added doc::ColorMode (equal to raster::PixelFormat)
- Added some methods to doc::Document: context(), sprites(), sprite()
  (to replace app::Document::getSprite()), width/height(), colorMode(),
  name(), and close()
- Moved app::DocumentObserver/Event to doc::DocumentObserver/Event
- Replaced app::ContextObserver with doc::DocumentsObserver and a couple
  of signals.
- Renamed app::Context::getActiveDocument() with
  doc::Context::activeDocument()
- Renamed app::Context::getActiveLocation() with
  app::Context::activeLocation()
- Removed app::ContextObserverList
- Removed app::DocumentId (now we can use doc::ObjectId)
- Removed app::Context::getSettings()