Create shorthand syntax for guiseq/keyseq

[shaunix]

We could create a shorthand for guiseq and keyseq that doesn't require nesting gui and key elements. For example, <keyseq>Ctrl+S</keyseq>. Basically, if there are no child elements, and there is a plus sign, split on the plus sign and treat the parts as child gui or key elements.

We should carefully check if this would impact any existing real-world documents, but I kind of doubt it.

[shaunix]

http://projectmallard.org/mep/mep0015