Planner: hook the gas combo box to a "focus-out" event handler

GTK's logic might be a bit flawed (or complicated) in terms of
"focus-out-event" and GtkComboBoxEntry objects as it does not work
by attaching said signal type directly to the GtkComboBoxEntry.
Perhaps it only makes sense for text input.

Since "focus-out-event" works for GtkEntry, we can retrieve the child
GtkEntry from the combo using gtk_bin_get_child(GTK_BIN(combo)
and attach the event handler to that.

This change should make it possible to update gas_model (GtkListStore)
when changing the widget focus with both the keyboard and mouse clicks.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>