qt: Add data provider for list of file names

* lang/qt/src/Makefile.am: Add new files and corresponding camel-case
header.
* lang/qt/src/dataprovider.h: Include interface from gpgme++ when
not building.
* lang/qt/src/filelistdataprovider.cpp,
lang/qt/src/filelistdataprovider.h: New.

The new data provider simplifies providing a nul-separated list of
UTF-8-encoded filenames, e.g. for creating signed or encrypted
archives. It is a simple read-only proxy for QByteArrayDataProvider.

• GnuPG-bug-id: T6342