- Make DeferredImageDescriptor, FileImageDescriptor, and
URLImageDescriptor adaptable to URL
- For FileImageDescriptor and URLImageDescriptor remove explicit
adaption to ImageFileNameProvider, since Adapters.adapt will also find
the inherited one
- Add new ImageDescriptor tests that validate URL adaption
- Refactor DeferredImageDescriptor to hold as field the URL from the
Supplier and get it only once
- Refactor MenuHelper#getIconURI by eliminating the reflection-based
part and by replacing the IAdaptable+IAdapterManager pattern by
Adapters.adapt, if no context is available, otherwise use the
org.eclipse.e4.core.services.adapter.Adapter as adapter.

Signed-off-by: Daniel Krügler <daniel.kruegler@gmail.com>