ADT: GraphicalEditorPart is the new GLE2.

ADT: GraphicalEditorPart is the new GLE2.

This CL represents the base for the new "Graphical Editor Part".

First, GLE2 has been renamed in GraphicalEditorPart. That's the
final name, I swear I won't change it again (until next month that is.)

The editor part has 3 composites: the top ConfigConfiguration
(same as usual, reused as-is), a new PaletteComposite and a new
LayoutCanavas. This last one displays the rendering image and
will deal with interactivity.

The LayoutCanvas is actually stacked with a label which displays
the rendering error. After a rendering, either the error or the
canvas is visible, depending on the success of the operation.
That would make it easier to have a different mechanism, for example
the error could be next to the last known rendering, they don't
have to be mutually exclusive.

It is worth noting that GraphicalEditorPart is 95% similar to
the GLE1, reusing all the glue code that we had to handle
layout requestes, refresh requests, sdk/framework load listeners,
configuration and file input changes, etc.

Both PaletteComposite and LayoutCanvas are currently embryonic
at best, just to make sure the editor part is structured correctly.

Change-Id: I36c2ae4d85a68e68a349adc63a718f06375e12c5