Split internal ImageHandle class into exposed API and internal methods

The ImageHandle class currently contains some methods that are supposed
to be exposed to other consumers (like GC) while others are only
supposed to be used inside Image. To this end, different method
visibilities are used. However, depending on where the ImageHandle is
used inside Image, only specific functionality should be accessible as
well. In particular, only the current lifecycle responsible for an
ImageHandle should be able to destroy that handle.

To better ensure this, this change splits the ImageHandle class into
multiple classes, such that only the intended functionality can be
accessed at different consumers.

Author: HeikoKlare