moderncv: update layout and example to version 0.6 of moderncv.layout

moderncv: update layout and example to version 0.6 of moderncv.layout
This means:
 - added subsection style in moderncv.layout
 (I had to promise to JMarc when I added the moderncv support to add this as soon as the new noderncv version is out.)
 - actualize preamble of modernCV.lyx to make it compile with the new version
 - better explanation of customization possibilities in the preamble of modernCV.lyx

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18808 a592a061-630c-0410-9148-cb99ea01b6c8