Insert menu accelerator 'x' now goes to box menu

Before, 'x' would go to "TeX Code" but that already has a shortcut
in the common bind files.

Reported and discussed on lyx-devel at:
https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg185082.html
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.08.23-00.00