kconfig: qconf: re-implement setSelected()

The default implementation for setSelected() at QTreeWidgetItem
allows multiple items to be selected.

Well, this should never be possible for the configItem lists.

So, implement a function that will automatically clean any
previous selection. This simplifies the logic somewhat, while
making the selection logic to be applied atomically, avoiding
future issues on that.

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