Write text async to selection clipboard

Looks like Atom Shell's clipboard API uses sync ipc calls on Linux which causes the writing of selected text to the selection clipboard to add a non-trivial amount of time to the selection change event dispatch.

https://github.com/atom/atom-shell/blob/879de423728832cece6e798e3cbbb46e61738a16/atom/common/api/lib/clipboard.coffee#L3

Regarding this now being async, in my testing, this didn't change the behavior of middle mouse paste on Linux. The selection was still written to the clipboard before I could hit the paste button quick enough to paste the wrong text. But if this does present issues it could wait until the text is written before pasting the text on middle click.

Closes #4238