Qt: Install CamelCase forward includes

* lang/qt/src/Makefile.am (camelcase_headers): New. Create and install
CamelCase headers.

For Qt Libraries it is a common pattern that headers are installed
additionally under their Namespace / Class Name so that automated
inclusion works once a class is used in code. This was also
done for QGpgME headers when they lived in Libkleo so this increases
compatibility.