Make Container widgets take children as the first positional argument

This makes creating containers less cumbersome: Container([list, of, children]), rather than Container(children=[list, of, children])
Update celltoolbar.js

I want to write an extension with multiple select bar. They are supposed to be displayed inline, not in block mode. The better way is to use span instead of div.
Container assumes the children attribute is not None

Thanks to @SylvainCorlay for pointing this out.
cell toolbar and modal dialog fixes

- name conflict in modal
- cell.cell_type in cell toolbar

prevented edit metadata from working properly.
Merge pull request #6153 from jasongrout/container-children-arg

Make Container widgets take children as the first positional argument
Add docs on writing kernels
Merge pull request #6127 from minrk/fix-edit-md

cell toolbar and modal dialog fixes
Merge pull request #6160 from parleur/patch-2

Span in CellToolBar instead of div
Add more see also entries
Merge pull request #6179 from takluyver/master

Add docs on writing kernels
Remove py3compat.open in favour of io.open
Fix excluded modules for docs
Don't try to autodocument fabfile
Merge pull request #6184 from takluyver/rm-py3compat-open

Remove py3compat.open
coalesce stream output in the notebook

This merges consecutive outputs on one stream into a single output.

Essentially, it applies the same merging that we do visually to the content stored in the notebook document.

This results in a massive performance improvement in load-time and storage size
for notebooks that have many calls to `sys.stdout.flush()`.
what’s new for coalescing stream output
test coalesced output in js
Avoid duplicate extension in nbconvert

nbconvert --to notebook --output Output.ipynb

would write Output.ipynb.ipynb before this PR
test `nbconvert --output` with and without extension
Merge pull request #6069 from minrk/coalesce_streams

coalesce stream output in the notebook