enhanced option to update same-named file

[flobolka]

usecase of the problem: updating a .pdf-file which has been already published&shared over a month ago via the media static URL (incl. the automatic year/month folder classification) would result in creating a new URL (year/month-folder of time when update is uploaded). Its not possible to update an existing file via the media manager with keeping the original URL.

possible solution: i.e. show an "overwrite"-option in the error/warning dialog which appears when you try to upload a file with an already used file-name. by choosing this option, the original file should be replaced without changing its folder/URL.