This made them fail when data was being written on standard outputs

Any host command (other than pause) will now be run as a pronsole
command. This will allow doing things such as #428 easily.

These 2100 lines of code were really a huge mess ><
Also rename some functions.

Better name will ease understanding the code :)

Heavily untested except on Arch Linux.

Calling clearhilight directly was probably leading to some exceptions
hidden by an try: except: pass block

This behavior was wrong for sequential prints and made little sense
since we already have a decent layer splitter in GCoder.

Also signal visualisation of a layer change from the appropriate
callback (layer_change_cb) instead of printsentcb.