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 !

This reverts commit 2a90d5b3.

Looks like a sed failure happened before ><

This is required to get correct drawing in Locked mode