Updating the coLaboratory UI to be cleaner and more like IPython.

Here’s a high level rundown of the changes:
 
- Moved the toolbar to the top-right of the cell, and added icons for most menu items.
- Focus and selection are now indicated by shadows.
- Outputs now have pictures instead of date and time, you can hover to get the date and time.
- You can add new cells in place by hovering in the margins between cells. 
- Clicking on output now selects a cell. 
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