qt: Add method for getting config entries for components by entry name

* lang/qt/src/cryptoconfig.h, lang/qt/src/cryptoconfig.cpp
(CryptoConfig::entry): Move implementation to cpp. Add overload not
requiring a group name.

The group name is not needed for identifying a config entry because the
groups only provide a logical grouping of config entry for user
interfaces. To improve usability entries are sometimes moved to a
different group, but this shouldn't break existing applications trying
to access those entries. The new group-agnostic overload makes
applications robust against regrouping of config entries.

• GnuPG-bug-id: T5217