This library provides an interface to implement import/export
plugins for KAddressbook. It was agreed that it is OK for
applications to install their own plugin interface libraries,
so moving the library from kdepim-apps-libs gets us one step
closer to killing that repo.

As part of the move I renamed files and classes and removed
KAddressbookImportExport from the name, since they are already
in KAddressbookImportExport namespace (which required adjusting
existing code in KAB.

The library has been also moved from KF5 to KPim namespace, since
this library is never going to become a Framework.