Add special paste behavior when copying/cutting w/ no selection

Addresses an issue pointed out by @dsandstrom in #4148.

@dsandstrom, @unDemian, if you wish to integrate this change into [clipboard-history](https://github.com/unDemian/clipboard-history), note that metadata field is called `fullLine`, not `fullline` as in @tekkub's [cut-line](https://github.com/tekkub/cut-line) package. I don't mean to be needlessly incompatible, but we always use camel-case identifiers, and I didn't want to break with that convention.