Merge pull request #884 from fperez/nb-usability Various Notebook usability fixes: - add kernel restart dialog confirmation (it destroys the user session) - add missing keybindings - change language from 'terminal mode' to 'in-place execution' which is more descriptive - adjust spacing/padding and colors slightly for readability and better use of space in small screens. - add line numbering toggle to code cells. Triggered with C-m-l. - show keybindings in monospaced font. - add QuickHelp button that is always visible and at the top. - add missing keybindings for kernel interrupt/restart from the kbd.