#1303 Add a compare mode button

[Pirella]

I use Winmerge mainly for comparing different versions of the same software. Usually I get the desired results.

However, some file types - like compressed javascript files - don't give useful results. Winmerge highlights lines with differences but as such files have everything on the same line you get the whole file highlighted. So I would like to be able to switch to a token mode where just the differences are highlighted.