The change from mbarcode-core 1.0.2-6 to 1.0.2-7 is a change to the libs section to avoid having the core package show up side-by-side with the mbarcode (meta) package in the package manager. I tried to use user/hidden for this, but it failed, so I guess we're stuck with libs if we want to promote it to Extras testing.

The dependencies changed because it became a blocker when pushing to Extras testing. Now the dependencies work this way:
mbarcode depends on mbarcode-core and some useful, well-working plugins. This package contains nothing.
mbarcode-core depends only on libraries needed for the executable. This package contains the executable, icons, etc.
All plugins, usually named mbarcode-plugin-*, depend on mbarcode-core. These contain plugin files.

mbarcode-plugins-all is discontinued and may safely be removed.

There is just one problem as I see it with the above solution, and that is that mbarcode-core probably won't be removed by the package manager if you uninstall mbarcode. I'm not sure if this is true, but its a minor problem and could in the worst case be fixed by putting mbarcode-core back into the user/* sections - thus making it visible in the application manager again.