linux: support 2nd clipboard

[gucio321]

Can this support middle-click-clipboard and copy-paste clipboard (Ctrl+C, Ctrl+v)?

[changkun]

Currently only the CLIPBOARD selection (the Ctrl+C / Ctrl+V clipboard) is supported. The PRIMARY selection (the middle-click / select-to-copy buffer on X11) is not implemented.

This is a reasonable feature request, so I'll leave it open and label it as an enhancement. Note it would be X11-specific — Windows and macOS have no equivalent of the PRIMARY selection.