Fix AHK script for Dropbox for other locales

Better i18n compatibility for the package: this should fix the issue with languages where Dropbox’s installer has a different window title than `Dropbox Setup`.

Now it matches any window that contains *Dropbox* in the title **and** which has the class `ahk_class #32770`. I’ve noticed that the window class remains the same across multiple computers and Windows versions, so I think it’s safe to use it. I hope that this is strict enough to not accidentally match the wrong window.