Keep the last 3 lastEdit.py files as backup

[butterw]

Avidemux auto-save the current project in settings/lastEdit.py.
When the edited file is closed, it can be restored through Recent > Restore Previous Session until you edit another file.
However when this happens or if restore fails (which will occur if the source file has been moved/renamed for instance, causing a tinyPy Exception) lastEdit.py is overwritten, which means the project is now permanently lost.

A much better approach would be to keep the last three lastEdit.py files and rename them rather than delete them.