Add a per cell toolbar.

This add a per-cell globally toggleable toolbar
the main purpouse is to easily edit metadata.

this come with a few example like adding checkbox,
dropdown list, simple button, button with icon...

please see the js-doc of
IPython/frontend/html/notebook/static/js/MetaUI.js
for more info
add utils and docs for metadata UI

add aconvenient utils to generate UI for checkboxes

and use it to generate 3 checkboxes for
new_section
new_subsection
new_fragment
Split metadataui into multiple file

Add example metadata UI into multiple categorized files.
And document how to load it using custom.js.
create metadata-dropdowm-menu generator
use dropdown for slideshow cell-type select
remove slideshow example
add a small default metaui-toolbar

just a question mark which pop-up a small text
enable dropdown preset change
convention, jQuery variable start with $

all this.xxxx = $(...) change to
    this.$xxxx

I'm thinking of potentially doing it also in parameters that are
expected to be jQuery enabled.
rename metaui -> celltoolbar
rename file metaui->celltoolbar
rename _button_list to _ui_controls_list
remove most of the duplicate example
remove relative position
do not use alert on invalid json
fix css size
remove useless jquery selector
slightly generalize utils generator
create celltoolbar in cell.js and inherit
change - to Hidden