add itemChooser plugin
updating default layout and elements

- adapt Common/admin_index
- include choose.js in admin layout
- include chooser field in default search element
change Links/admin_form to use chooser
updating Node chooser implementation

also renames Nodes/admin_links to Nodes/admin_chooser
implement Block chooser
change Attachment/browse to use admin_popup layout
implement User chooser
adding chooser usage example