Document improvements for the keyboard shortcuts plugin module (READY TO PUBLISH)

There's a couple of improvements in the keyboard shortcuts module. Namely:

• Plugin writers can now hide keyboard shortcuts from the shortcut help dialog by adding a hidden='true' attribute to the XML definition
• Plugin writers can overwrite existing keyboard shortcuts. To do so they simply have to define a shortcut module which defines the same shortcut keys (i.e. the same <shortcut> section) and with a higher <order> field.