Add notion of action that differs from  shortcuts.

This decouple the notion of shortcut from the notion of executed "action"
This allow the shortcuts manager to be purely describe as data, and the
same action to be later refered to either from the shortcut, from a toolbar
button or a menu.

This also implement a more complete keyboard shortcut handler which is
able ton interpete sequences like `Cmd-X,Meta-v` By storing the
shortcuts in a tree.
remove debug
add test warning shadow
Tooltip on toggle button
Make favicon and logo images blocks in the template
Fix #6385
initial take on promises
Initial stab at adding promises to the widget framework.
Fix view rendering order.
Add Promise support to models.
Use load_class in comms instead of callbacks.
Proxy console.error calls
Simplify code by using Promises in a better way; try_load -> load
More simplifications due to promises
Add some error handling for creating views and models
Better error messages with correct stack traces
Dictionary key/value typo
Load the utils module
Fix some this vs. that errors, as well as returning some promises to wait for fulfillment
Make display also pend on set_state.