Allow running complete_done_after() in normal mode

If user selects a completion item and presses <Esc>, the key sequence to feedkeys() will get run in normal mode. This can be handled with a <Plug> mapping that resolves to different sequences in normal mode and insert mode.

As a bonus, the use of <silent> mappings eliminate the message line garbage from feedkeys.

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@infmagic2047 could you fix merge conflicts.

I just rebased on master.

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.