This is the fix for the regression introduced in 4d596ad while changing
from PDE own `OverlayIcon` to `DecorationOverlayIcon` from JFace.

The `SharedLabelProvider.createImage(ImageDescriptor baseDesc, int
flags)` which originally created only one `Image` instance for image
with decorations (which was properly stored & disposed if
`SharedLabelProvider` was disposed), now creates **two** `Image`
instances, one from which (for the base image) always leaks because
`DecorationOverlayIcon` never disposes base image.

Moved the responsibility to "remember" created images down to the code
that creates images, and added extra code to remember base image too.

Fixes https://github.com/eclipse-pde/eclipse.pde/issues/2211