Remove prontserve.py

How many prontserve leftover did I miss ? ^^
Move setlayer() call to printsentcb
Rename gviz.addgcode to addgcodehilight, remove non-highlight codepaths
Rework gviz to avoid indexing layers by their Z

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.
Factorize code in gviz
Move clearhilight() call to gviz.setlayer()

Calling clearhilight directly was probably leading to some exceptions
hidden by an try: except: pass block
Report more exceptions in printcore callbacks
Add option to track currently printing layer in 3D view (#487)
Merge branch 'master' of github.com:kliment/Printrun