proportional font-size when editing heading cells

@MinRK @takluyver and I had the idea to do this when we were discussing #6225.
appropriate heading level when loading from JSON
match font family when editing heading cells
fix more style consistencies
Fix qtconsole input prompt accounting

- Ensure prompt text is flushed after showing
- Ensure prompt position is adjusted when reading input
- Keep pending text to insert only when inserting at end

This fixes `ipdb>` input prompts when debugging a file
in qtconsole.
Merge pull request #6245 from pankajp/fix-qtconsole-prompt-accounting

Fix qtconsole input prompt accounting
Merge pull request #6238 from ivanov/cm-heading-fonts

proportional font-size when editing heading cells