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 ? ^^

UI state is not preserved yet (controls will be recreated as disabled,
button state won't be restored)

Previous rename was wrong as it overlapped with another variable in use,
thus breaking everything. D'oh !