Add numeric IDs to CatalogItem.
Display item IDs instead of line numbers.

Instead of completely removing the confusing line numbers, as was the
plan for 1.5, replace them with unique IDs.  This avoids the confusion
of mismatching lines, yet still provides a way to identify individual
entries.  It also opens the way to using symbolic IDs (as is sometimes
done even in gettext).

The plan is to eventually move this to the first column and always use,
instead of being configurable, but that's more work due to various
problems in wxListCtrl.  So this will suffice for now and better UI will
follow after the migration to wxDataViewCtrl.
Reorganize files in Xcode project.

Group them semantically, not by headers/source file type.
Fix TM in Visual Studio builds.

Berkeley DB had incompatible CRT settings... and USE_TRANSMEM was
accidentally left undefined, so the translation memory was omitted from
the Windows build completely.