It was possible to deselect items when using a touch screen. This is done
by tapping on an item that's already selected. This would cause a segfault
in the select_item() function of src/wofi.c since
gtk_flow_box_get_selected_children() would return NULL in this case. By
setting the selection mode to GTK_SELECTION_BROWSE, we avoid this case
by ensuring an item is always selected.
