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.

How many prontserve leftover did I miss ? ^^