kconfig: qconf: convert to Qt5 new signal/slot connection syntax

Now that the Qt4 support was dropped, we can use the new connection
syntax supported by Qt5. It provides compile-time checking of the
validity of the connection.

Previously, the connection between signals and slots were checked
only run-time.

Commit d85de339

 ("kconfig: qconf: fix signal connection to invalid
slots") fixed wrong slots.

This change makes it possible to catch such mistakes easily.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Boris Kolpackov <boris@codesynthesis.com>