ui-tree.c: Use a context structure in walk_tree()

Use the context pointer to pass context information instead of misusing
global variables, as we already did in "ui-blob.c" and in "ui-plain.c".

In addition to the fixes to walk_tree(), pass the same structure to
ls_tree() and ls_item() which is read_tree_recursive()-based as well.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>