Fix installation of qt.conf with MSI installers

* src/Makefile.am (EXTRA_DIST): Add qt.conf.
* src/inst-qtbase.nsi: Install qt.conf file instead of writing it.
* src/qt.conf: New.

When the environment variable GNUPGHOME is set this returns
the according values using QStandardPaths.

This was not applied for GnuPG VS-Desktop since it used
the FileWrite mechanism of NSIS instead if installing the
file, which is not converted into WXS.

(manually cherry picked from 70175060e8d01c313968d02b915f9dc47b1ca69d)

• GnuPG-bug-id: T7700