Add new shortcuts and some fixes for Chrome Dev Tools cheatsheet

Hi,

These commits:
- clarify usage of commands in Elements panel
- correct use of next/previous commands in Styles panel
- add new commands.

Commands are tested on 10.9 with Dash 2.0.1, built from repo and installed manually in Dash. After each commit *.rb is rebuilt and reloaded in Dash to verify it works ok.

Peter