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/slic3r/GUI/UpdateDialogs.hpp

@@ -5,7 +5,7 @@
 #include <unordered_map>
 #include <vector>
 
-#include "slic3r/Utils/Semver.hpp"
+#include "libslic3r/Semver.hpp"
 #include "MsgDialog.hpp"
 
 class wxBoxSizer;