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
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