Include $DISPLAY in cache dir path

[SabrinaJewson]

Since clipmenud is specific to one X $DISPLAY, that value should be included in the path to its cache directory. This would allow correct handling of multiple X displays for one user, where each display would have its own clipmenud instance running separately.

[cdown]

That's actually a good point, thanks. This will require bumping major_version, though, so I'll save it for when we have more changes to go since it's not a problem that seems very widespread (or at least, I've never heard a report about it).