add illustrative comment in toplevel_child.c
test panel widget in toplevel_child

Thinking about using panel in tab to display sub-widgets on the current
tab-page. Testing if panel widget is completely implemented and working.
fix panel on passing event messages

Panel widget was passing the original message to sub-widgets in show()
function, but the original message in this function can be of another
type like mouse press. This behavior causes sub-widgets to act like
panel itself even if mouse is out of bound.