Fix use-after-free in ConfigureAppearancePage

KLocalizedString().untranslatedText().constData() returns pointer
to data of a temporary QString that gets destroyed at the end of
the statement. Accessing the data in the pointer later accesses
a free'd memory. ASAN has caught this as heap-use-after-free.

I don't see a reason to use constData(), especially since we only
use it to construct a QString from it later on, so we can just use
the QString right away.