Add a "Open Existing File" to edit existing file dialog

[sunstorm1000]

When one edits a file from the server and the file is already being edited, it would be nice if the dialog that asks "A file with that name is already being edited. Discard old file and download the new file?" also had an "Open existing file" button, rather than just a Yes or No.

The existing file is usually sitting somewhere in a hard to reach location, and I might not want to redownload it, such as in the case where I closed the editor by accident.

[Tim Kosse]

Will be implemented in next version.