Install headers to path prefix matching C++ namespace KCDDB

Also use new header visibility protection prefix KCddb5,
as sadly on case-insensitive filesystems there will be a clash
between the c++ namespace subfolder "KCDDB/" and the old camelcase forwarding
header "Kcddb"

Backward-compat headers are provided supporting version-controlled
warnings & disabling reusing the macros provided by
ecm_generate_export_header

There is no backward-compat file for kcddb_export.h & configbase.h,
it is assumed no 3rd-party explicitly includes those directly.