WIP: Moved sources int src/, separated most of the source code from Perl.

The XS was left only for the unit / integration tests, and it links
libslic3r only. No wxWidgets are allowed to be used from Perl starting
from now.

src/slic3r/GUI/Widget.hpp

@@ -0,0 +1,16 @@
+#ifndef WIDGET_HPP
+#define WIDGET_HPP
+#include <wx/wxprec.h>
+#ifndef WX_PRECOM
+#include <wx/wx.h>
+#endif
+
+class Widget {
+protected:
+    wxSizer* _sizer;
+public:
+    Widget(): _sizer(nullptr) { }
+    bool valid() const { return _sizer != nullptr; } 
+    wxSizer* sizer() const { return _sizer; } 
+};
+#endif