Provide additional default values (directories at other Dired

Provide additional default values (directories at other Dired
windows) via M-n in the minibuffer of some Dired commands.

* dired-aux.el (dired-diff, dired-compare-directories)
(dired-do-create-files): Use `dired-dwim-target-defaults' to set
`minibuffer-default' in `minibuffer-with-setup-hook'.
(dired-dwim-target-directory): Find a window that displays Dired
buffer instead of failing when the next window is not Dired.
Use `get-window-with-predicate' to find for the next Dired window.
(dired-dwim-target-defaults): New function.

* ediff-util.el (ediff-read-file-name):
Use `dired-dwim-target-defaults' to set `minibuffer-default'
in `minibuffer-with-setup-hook'.