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/Semver.cpp

@@ -0,0 +1,7 @@
+#include "libslic3r.h"
+
+namespace Slic3r {
+
+Semver SEMVER { SLIC3R_VERSION };
+
+}