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).

[cdown]

I've been thinking about this for clipmenu 7 and I'm not entirely sure that it would interact well for cases where uses X forwarding, right? Then one gets a completely different clipmenud, even though it's attached to the same eventual X display.