Updated to improve support for OctoPrint use

The pe_lmsg and pe_rmsg commands have been superseded by pe_imsg, which takes up to 20 characters and will be displayed split 10 chars left/right as appropriate, or just the first 10 characters on the right in 'busy' mode.

A new control pe_bcfg has been added to control how the button behaves; giving options for M112 (reset/emergency stop) and other uses. As well as Octoprint-friendly pause/resume in busy/idle states.

See the README for more, it has been updated with the new commands and an example of how to get progress display from Slic3r. Getting 'native' OctoPrint progress display is tricky and I really need to write a plugin to handle that.

The code is a bit smaller too thanks to some refactoring and learning to better use progmem for fixed string functions.