MEP: Shorthand syntax for guiseq and keyseq

http://projectmallard.org/mep/mep0015.html

In short, instead of this:

  <keyseq><key>Ctrl</key><key>S</key></keyseq>

Write this:

  <keyseq>Ctrl+S</keyseq>

And instead of this:

  <guiseq><gui>File</gui><gui>Save</gui></guiseq>

Write this:

  <guiseq>File+Save</guiseq>

The plus feels a little weird in guiseq, but very natural in keyseq. I
think there's value in using the same character in both. Change my
mind.

--
Shaun