Patch from Frederic Bouvier:

Patch from Frederic Bouvier:

I have modified the SGPropertyNode interface to support node removal
and address the issues you mention yesterday.

I added the SGPropertyNode_ptr class that is a smart pointer that
increment and decrement a counter in SGPropertyNode. The _children
vector is changed to a vector<SGPropertyNode_ptr> in order for the
node to hold a reference to its children. I also added a
_removedChildren vector of the same type. When removeChild is called
with the keep parameter set to true, the reference is removed from
_children and inserted in _removedChildren, and the attribute REMOVED
is set. When getChild is called with create set to true, the _children
vector is searched, then the _removedChildren and then a new node is
created. If a resurrected node is returned, the REMOVED bit is cleared.
The static const int LAST_USED_ATTRIBUTE is also added.

The footprint of this patch is light. SGPropertyNode is one int
longer in size, SGPropertyNode_ptr is one pointer large without
virtual functions. Some functions can be inlined if your current
policy change.

The FlightGear patch is to take account the change in the getChildren
function that now returns a vector<SGPropertyNode_ptr>. If the
removeChild functionnality is to be added in FlightGear, all those
SGPropertyNode * floating around should be changed to
SGPropertyNode_ptr.