Support clipboard editing

[lf-]

Not sure if this is a feature-creep thing or out of scope, but it would be nice if x11fs supported changing and viewing the clipboard buffers.

Say, ~/.x/clip/{primary,secondary,clipboard}.

[halfwit]

I really like the idea personally.

[sdhand]

Apologies for taking so long to respond, I've been rather busy the past few days.
I can see how this could be useful, but am worried about feature-creep and filling the filesystem with too many "unnecessary" items. I'm going to implement this in a separate branch for now, but I may end up merging it into main branch if things turn out well.

[halfwit]

I am starting work on this, but I have no eta for how long it will take, as I'm heading into my busy season soon.

[halfwit]

(For the record, libclipboard now exists; hopefully that can be used to do a much better job than what I had)