button: Clear highlight when a button is removed from a container

button: Clear highlight when a button is removed from a container

A button is highlighted if the private variable in_button is TRUE.
This variable is set when the pointer is over the button and cleared when
it left the button. When a button is hidden while there is the pointer over
it, GTK generates a leave notification event, in_button is set to FALSE.
But when a button is removed from a container but not destroyed, it is
unrealized and loose its window. It cannot receive the leave notification
event and in_button stay TRUE. So when the button get a new parent it is still
highlighted.

https://bugzilla.gnome.org/show_bug.cgi?id=676890