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.
Translate PlaceholderParser::apply_config to C++.
Merge pull request #2091 from sapir/cpp_placeholder_apply_config

Translate PlaceholderParser::apply_config to C++.
Merge branch 'call_get_layer' of github.com:sapir/Slic3r into sapir-call_get_layer

Conflicts:
	lib/Slic3r/Print.pm
Merge branch 'sapir-call_get_layer'
Fix tests
Added min limits for most numeric options