Fix 'Font is always bold when model is opened, although default font is regular style'

Umbrello modifies font style to indicate specific class/member attribute
states, so we need to disable any font style preferences.

Because QFontDialog does not support disabling font style list, it is
required to fall back to KFontChooser for the KF5 variant.

BUG:389357
FIXED-IN:2.24.2 (KDE Applications 17.12.2)