Clear clipboard state when its not plain text

Clear clipboard state when its not plain text

Commit 8a80c906b9f7c08b1c664569a97d327d62c6d8ae added a check to prevent
announcing clipboard when it's not plain text data. When this check
triggers and we want to ignore the non-plain text clipboard, we should
reset the state by no longer indicating pending clipboard, and by asking
the server to clear its clipboard. This ensures we don't end up in a
state where the server thinks the viewer has useful clipboard data, but
it doesn't.

(cherry picked from commit 6f6d9406035d59b6a9f59ff0f38206c0c98a0266)