Cleanup tree.c cache handling a bit.

 * Rename stuff to not re-use the term dircache
 * Move cache to own struct
 * Encapsulate retrieving entries a bit

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30242 a1c6a512-1295-4272-9138-f99709370657
9 files changed