Clean Addons (but leave config) Button
Description A button to delete all addon .dll files and the addon loader .dll, leaving everything else in /addons (i.e. config) in place.
Implementation UI: I would put this as the option immediately preceding the clean install button, and maybe rename both for clarity. Not sure what the most clear wording would be.
Backend: This should delete all addon .dll files and the addon loader .dll. It should also either delete the version information out of config.yaml, or maybe delete config.yaml completely (haven't thought that through too much).
Alternatives Right now this is fairly difficult to do if needed (i.e. you manually replaced some of the .dlls and now the addon manager is confused about what files are actually there). Changes to the detection code could prevent this ever being required, in theory, but I don't know that it's possible to guarantee things never get borked.