Convert QStrings with qUtf8Printable for GTK API calls

The fix for dangling pointers in 524f39 caused some problems when the
QByteArray was implicitly converted to const gchar*. This is fixed
by wrapping the QString in question in qUtf8Printable where possible
and removing the former convenience method.

Task-number: QTBUG-59692
Change-Id: I5abcf42e1c23b12c7a5c4c195d801f377fe9d138
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>