Refactor: Move Semver from slice3r to libslic3r

A static symbol Slic3r::SEMVER is introduced, which holds
the running slicer's Semver object.

This is mainly done to make testing updater behaviour
_much_ easier. Additionaly to cleanup some questionable code
(Semver was being parsed multiple times / in multiple places
in the frontend.)

src/libslic3r/CMakeLists.txt

@@ -139,6 +139,7 @@ add_library(libslic3r STATIC
     PrintConfig.hpp
     PrintObject.cpp
     PrintRegion.cpp
+    Semver.cpp
     SLAPrint.cpp
     SLAPrint.hpp
     SLA/SLAAutoSupports.hpp