keyboard: add more sequences, from existing ones or RFC 1345

R=rsc
CC=plan9port.codebot
https://codereview.appspot.com/43990046

Committer: Russ Cox <rsc@swtch.com>
INSTALL: diagnose missing gcc on Darwin

TBR=rsc
https://codereview.appspot.com/55700043
devdraw: clear altdown on focus out (thanks Ethan Burns)

TBR=rsc
https://codereview.appspot.com/53820044
mac/Plumb: add URL scheme to Info.plist

LGTM=rsc
R=rsc
CC=plan9port.codebot
https://codereview.appspot.com/40780044

Committer: Russ Cox <rsc@swtch.com>