Allow setting value of primary clipboard buffer on linux

[nightwing]

On most Linux distros middle click pastes last selected text.

For web editors that do not render all the visible text, or do not use browser selection, this behavior is impossible to replicate ajaxorg/ace#175 codemirror/codemirror5#931

On Chrome it is possible to use a hack, by putting the text into a hidden textarea and selecting it, but that causes performance issues, and doesn't work on other browsers.

It would be useful to either have a performant way to set the value of primary clipboard, or a way to set it when editor is blurred.

The old, non-web clipboard api on firefox provided a way to do this.