headerbar: Disconnect signal handlers when children are removed

headerbar: Disconnect signal handlers when children are removed

The signal handler that calls notify_child_cb() is disconnected
from a child widget before the child is removed from the header bar.
gtk_header_bar_dispose() chains up before destroying the start and end
boxes, thus avoiding calls to notify_child_cb() after start_box and
end_box have been cleared.

Fixes #2246