Improved support for running on a real VT340

* Redesigned flag sequence page with left-to-right layout.
* Fixed image rendering glitches on double-width lines.
* BkSp supported as an alternative to Esc for returning to the main menu.
* Ctrl+BkSp supported on the flag sequence page to reset the input.
* Re-enabled the IXON flag to allow for software flow control.

Thanks to @hackerb9 for help finding and fixing these issues.

Initial release