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
Public server firewall configuration

Added section on firewall configuration.  This should prevent users (like me) from struggling
to figure out why their servers aren't executing code.
Fixed port range

Actual port range for ZMQ is 49152 through 65535.  Also fixed some grammar/spelling issues.
Allow starting the server with both file_to_run and notebook_dir

file_to_run and notebook_dir would collide, with possible dictionary
ordering randomness depending on how they were passed.

With this change, the default value of notebook_dir is pulled from
file_to_run, instead of pushing the value when file_to_run is changed.
This makes it possible to specify both and have the server behave as
expected.
Allow FileContentsManager and MappingKernelManager to be used outside NotebookApp
A Union Traitlet
Allow for trait types like Type or Instance
Test for Union Traitlet
validate -> _validate
resolve classes
Overload __or__ on traitlets and add a test
Remove comparisons in traitlet links
Some cleanup unused code and missig use-strict
update to use event
some extra comma/semicolon cleanup
Merge pull request #6925 from SylvainCorlay/union

A union traitlet type
Merge pull request #6748 from Carreau/cleanup-nb

Some cleanup unused code and missing use-strict
Merge pull request #6989 from SylvainCorlay/safe_link

Remove comparisons in traitlet links and directional links