Fix listbox scrolling

Long term bug with listbox scrolling fixed and added an example for use over Classic Bluetooth Serial on an ESP32. Which seems like a reasonable use case for an infrequently used configuration interface in a project while not being yet another a captive web portal.

Fix #10 compilation error on AVR

Simple cut/paste failure, had only been tested on ESP32 sorry folks.

Fix memory leak

There had always been a memory leak due to some bad bitwise logic, this is now fixed. It was only noticeable if you repeatedly changed the content of a widget.

Add callback support

Added the option to use callbacks instead of polling each widget for clicks. This massively improves 'idle' performance when there are a lot of widgets. Other small fixes, see the changelog.

Markdown header bugfix + heap fragmentation avoidance

Fixed a bug in parsing markdown headers. Also small improvements in markdown formatting and widgets now only re-allocate memory for changed content if necessary.

First bugfix

Single letter widget shortcut keys would cause a crash so an immediate version update seemed appropriate.

First public release

The first public release of retroTerm should be stable enough for experimentation. As it is a pre-release the API is not settled yet and may change.