ImageViewer: De-pimpl two internal classes

Change-Id: I3302463157f757fbb5190f25052fad1506d10612
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
DiffEditor: Remove DescriptionEditor helper class.

With value-based ids it's just a normal BaseTextEditor.

Change-Id: Ie9d28a92c48cb222eb9173bd432f08f4fe628bed
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>