API: Allow NotebookManagers to control kernel startup dir. #5468
cleanup some `pre` css inheritance

We had various overrides of the bootstrap defaults for font-size and line-height every time we made a pre tag,
and then some overrides of our overrides when we wanted something slightly different.

This moves the most basic overrides to apply to all pre tags,
reducing the number of downstream changes we need.
support binary files in FilesWriter
add PDFExporter

replace `nbconvert --to latex --post pdf`
with `nbconvert --to pdf`
support pdf export in the notebook UI
Added an optional parameter to "CellToolbar.register_callback".

It lists the cell types the UI element must inserted in.
Hide the cell toolbar if there are no ui controls in it.
Hide toolbar of rendered text cells.
Why overriding a method if the new one is exactly the same of the one
in the base class?
Removed "cell_types" parameter from "select_ui_generator".

If you don't want to show the UI element in a cell use the
"register_callback" optional parameter.
Adds markdown formatting to output cells
Update bundled pexpect to 3.2

Closes gh-5679
Do not record memory addresses of display objects in repr
Add test for DisplayObject.__repr__
Test against object.__repr__, rather than hardcoded repr
Fix non-ascii spaces in comment.

Closes gh-5697
Merge pull request #5648 from andrewjesaitis/ticket4756

Adds markdown formatting to output cells
Merge pull request #5644 from jhamrick/master

Do not record memory addresses of display objects
remove PDF post processor
note removal of --post pdf in whatsnew