kconfig: qconf: fix the content of the main widget

kconfig: qconf: fix the content of the main widget

The port to Qt5 tried to preserve the same way as it used
to work with Qt3 and Qt4. However, at least with newer
versions of Qt5 (5.13), this doesn't work properly.

Change the schema by adding a vertical layout, in order
for it to start working properly again.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>