IPython fixes
This PR contains couple of fixes for the IPython console plugin, addressing the following issues under IPython 7.11:
- an error when multi-line input results in (auto)indentation
- an error when user tries to clear current buffer via keyboard interrupt (Ctrl+C)
- an error due to invocation of deprecated (and removed) autoedit_syntax
- under certain conditions, auto-complete attempt clears all characters from the input but the last one (e.g., "pr" + Tab results in only "r" remaining)