This model is responsible for loading the memory data and observation
ranges. This is a refactoring change, which moves the RPC logic out of
the view into a model class. As a side-effect, fixes some timing issues
in the memory view.

Also: collapses observation, read and write ranges so that the observation
drop-down doesn't explode on draw calls that read from the application
pool and the read/write highlights don't have random gaps.

Renames the Thumbnails model to ImagesModel (Images is already used in
multiple places, sigh) and gives it the ability to load textures and
framebuffers.