allow the location of qt.conf to be specified at runtime

Currently, Qt reads the qt.conf file from the location of the application executable. It would be nice if a different location could be specified instead, perhaps via an environment variable (e.g. QT_CONF_FILE).

Reading it from the location of the application executable is a problem for languages like Python and Julia that load Qt dynamically (and which might load different Qt libraries at runtime depending on how the paths are configured). For example, in Julia we are calling the Python Matplotlib library for plotting, and this in turn loads Qt dynamically via the PyQt5 module. However, Python's qt.conf file is in the location of the Python executable, which is different from that of the Julia executable. Of course, there are workarounds like copying Python's qt.conf to the location of the Julia executable, but this has various shortcomings (the Julia language itself is not shipped with Python or Qt, which are loaded dynamically by a package, the Julia executable may be in a read-only location, and different Julia programs may link to different versions of Python and/or Qt).