'mpe mode': click to copy patch IDs

'mpe mode': click to copy patch IDs

If 'Show Patch IDs' is turned on in settings, add an extra column
to the patch list, with buttons showing the patch IDs. The buttons
copy the patch IDs to the clipboard.

JavaScript inspired by https://github.com/Triforcey/clip-j and many
many StackOverflow answers.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>