Refactor to make model rendering reusable

Refactor to make model rendering reusable

This changeset refactors the code in GraphicalEditorPart a bit to make
the part which renders a model into an image reusable. This will be
used by an upcoming changeset to provide previews of palette items as
you drag them.

The refactoring is straightforward, even though the diffs
unfortunately don't show it. I basically split up the large
recomputeLayout() method into smaller separate chunks (validating the
current file, validating the model and validating the sdk and looking
up the bridge) and added a new render method which can take just a
model (separate from the editor's own model) and render it.

Change-Id: I96603c4beb2d56d36416cd2b6448c87dd306217c