change font size in menus and widgets

[cccplex]

I installed Arch Linux and as this is a barebone OS I had to find a way to configure the font size. Mine was too big.
Setting the value for QT_AUTO_SCREEN_SCALE_FACTOR only changes the size of borders and buttons.
Changing font size in qt5ct didn't do anything for me. But setting the DPI value for QT_FONT_DPI finally did the trick.
So i have `export QT_FONT_DPI=96` in my `.xsession` file but you can also put that in the `qcad` shell script in the qcad folder.