Make update_tree_view() work faster on single-parent, many folders tree.

Make update_tree_view() work faster on single-parent, many folders tree.

    Make update_tree_view() again use the technique which remembered the
last added item, and if it did not work - moved up until the top of the
tree is reached. In case this technique does not work - fall back to the
"locate-and-add" slow method of adding sub-folders/tree items. The
current used on can be deteremined when kmk is started with debug level
+4 (third bit of a byte): message for fast adding says "FAST adding ..."
and fall back is showed by "doing a slow search..." message followed by
a message that says only "adding ..."