kconfig: gconf: use configure-event handler to adjust pane separator

The size-request event handler is currently used to adjust the position
of the horizontal separator in the right pane.

However, the size-request signal is not available in GTK 3. Use the
configure-event signal instead.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>