DiffEditor: Fix warnings on clang/msvc

The recent move to use classes in favor of structs missed the forward
declarations, so the build produces warnings about the mismatch when
using clang (and MSVC as far as I can tell).

Change-Id: I235f7410a5b43b14c318843c9ee4dfcb9ea88e63
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>