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/MsgDialog.hpp

@@ -8,8 +8,6 @@
 #include <wx/font.h>
 #include <wx/bitmap.h>
 
-#include "slic3r/Utils/Semver.hpp"
-
 class wxBoxSizer;
 class wxCheckBox;
 class wxStaticBitmap;