Implemented avoid_crossing_perimeters with VisiLibity
Update MANIFEST and test
Merge branch 'master' into visilibity

Conflicts:
	lib/Slic3r/GCode.pm
	xs/MANIFEST
Fix MANIFEST
Simplify islands before initializing MotionPlanner
New implementation of Avoid crossing perimeters using Voronoi diagrams
Partial work for background processing
A few improvements to gcode placeholder tests.
More work for background processing
Translate PlaceholderParser::apply_config to C++.
Added an option for enabling/disabling background processing
Removed warning
Call get_layer instead of layers->[] (more efficient now that layers() recreates the array).
Fixed concurrency issues
Refactoring: better API for Print/Object steps with explicit dependencies
Minor fixes to make_skirt and make_brim
Use async timer everywhere
Don't invalidate things if the changed options only affect G-code export
Suspend background processes while writing to shared data structures
Merge pull request #2090 from sapir/improve_placeholder_tests

A few improvements to gcode placeholder tests.