kconfig: gconf: update pane correctly after loading a config file

Every time a config file is loaded (either by clicking the "Load" button
or selecting "File" -> "Load" from the menu), a new list is appended to
the pane.

The current tree needs to be cleared by calling gtk_tree_store_clear().

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