Better dialog to save attachments.

This commit replaces the call to `read-directory-name' by a call to
`read-file-name' when asking the user about the target location for
saving an attachment. This new dialog is better suited because:

1/ the user is currently saving a file, not a directory: he might want
   to complete over existing files (e.g., to replace an existing file
   with a new one just received).

2/ the current dialog was asking for a directory but then, if the user
   enters a directory, mu4e proposes him to "override" the directory
   with a file. This is not going to work obviously. In the new dialog,
   it is also possible to enter a directory name instead of a file name.
   In this case, a new file is created under the chosen directory name.