kconfig: qconf: refactor icon setups

These icon data are used by ConfigItem, but stored in each instance
of ConfigView. There is no point to keep the same data in each of 3
instances, "menu", "config", and "search".

Move the icon data to the more relevant ConfigItem class, and make
them static members.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>