send_file: allow filename for inline content-disposition

[ThiefMaster]

send_file should support attachment_filename even for content-disposition inline.

Including a filename with inline files makes perfect sense e.g. when serving images users may want to "rightclick -> save as".
To avoid breaking API changes I would recommend keeping attachment_filename for it, but sending a content-disposition:inline header in case as_attachment is false. Alternatively we could deprecate attachment_filename in favor of a new client_filename, where the old kwarg would not add an inline disposition while the new one would.

This depends on #1850 since it moves send_file from Flask to werkzeug.

[davidism]

Since this is essentially a fresh start, we can just rename it client_filename with the behavior described. Flask's send_file will still exist as it has to get various Flask-specific data, so it can do the deprecation there.

If client_filename is set, set disposition to inline, unless as_attachment is set too, then set disposition to attachment.

[davidism]

download_name seems like a clearer description of what the parameter does.