GTree: add an ability to iterate over a tree and a node-based API

The basic API that this commit adds allows in-order iterating over a GTree.

For this the following API were implemented or exported:
1) Returning the first or the last node in the tree,

2) Taking a pointer to a node in the tree and returning the previous or the
next in-order node,

3) Allowing to do a binary search for a particular key value and returning
the pointer to its node,

4) Returning the newly inserted or set node from both insert and replace
functions, so this node is immediately available and does not have to be
looked up,

5) Traversing the tree in-order providing a node pointer to the
caller-provided traversal function.

Most of the above functions were already present in the code, but they
returned the value that is stored at a particular node instead of the
pointer to the node itself.

So most of the code for these new API calls is shared with these existing
ones, just adapted to return the pointer to the node.

Additionally, the so called "lower bound" and "upper bound" operations
were implemented.

The first one returns the first element that is greater than or equal to
the searched key, while the second returns the first element that is
strictly greater than the searched key.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

docs/reference/glib/glib-sections.txt

@@ -2982,21 +2982,36 @@ g_bytes_get_type
 <TITLE>Balanced Binary Trees</TITLE>
 <FILE>trees-binary</FILE>
 GTree
+GTreeNode
 g_tree_new
 g_tree_ref
 g_tree_unref
 g_tree_new_with_data
 g_tree_new_full
+g_tree_node_first
+g_tree_node_last
+g_tree_node_previous
+g_tree_node_next
+g_tree_insert_node
 g_tree_insert
+g_tree_replace_node
 g_tree_replace
+g_tree_node_key
+g_tree_node_value
 g_tree_nnodes
 g_tree_height
+g_tree_lookup_node
 g_tree_lookup
 g_tree_lookup_extended
+g_tree_foreach_node
 g_tree_foreach
 g_tree_traverse
 GTraverseFunc
+GTraverseNodeFunc
+g_tree_search_node
 g_tree_search
+g_tree_lower_bound
+g_tree_upper_bound
 g_tree_remove
 g_tree_steal
 g_tree_destroy