[OS X] font issues 2: font styles get mixed up

NB: I wrote the description that follows based on Qt4, but it can easily be shown that Qt5 is just as affected by the issue (see under "Environment").

Qt4 cannot handle font styles reliably that do not adhere to the most commonly used style names (normal/regular, bold, italic, bold-italic). Selecting another style will work, but it would seem that reverse lookup to obtain the style name from the internal representation fails. For fonts with semibold and bold typefaces, semibold will become bold, for example.

Attached are screenshots from qtconfig's font selection panel. The 1st qtconfig pic is built and runs against qt4-mac from MacPorts, the 2nd against an official 4.8.1 build.
The selected default font is Lucida Sans Regular, which in this case also is the font shown in the preview when relaunching qtconfig and opening the font panel. But the style selector claims this is Demibold Italic. Changing anything else in the config will also save this font setting.
With a font family like Segoe UI or Adobe Source Sans Pro, selecting semibold leads to bold, and then anything displaying text in default font+bold will use the Black style.
The issue can also be seen by selecting Semi-Bold fonts for the browser and the application in the Assistant's settings (Qt4 & Qt5).

Contrary to the 1st font issue I just reported (https://bugreports.qt-project.org/browse/QTBUG-41710), this bug also plagues KDE applications.

NB: I've added 4.8.7 to the version list above (in both tickets) because I expect them to be there too ... unless someone cares to fix them.