Maintain a list of files marked for deletion while refreshing a dired

Maintain a list of files marked for deletion while refreshing a dired
buffer. Previously, when refreshing the buffer the files marked for
deletion would be lost.

Since the relationship between the files that have been marked for
deletion and those that exist on disk is volatile, I have chosen to
implement the discovery of marked files during the refresh function as
opposed to maintaining a dired buffer specific list.