kconfig: qconf: use QString to store path to configuration file

kconfig: qconf: use QString to store path to configuration file

This is the native type used by the file dialogs and avoids any hassle with
filename encoding when converting this back and forth to a character array.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>