Open the Design tab quickly on Windows the first time

A page built on first click is now built before it is shown, as the idle
prebuild already does, so its controls are not created inside a visible
window. This takes the Design tab's first open on Windows from ~9 s to ~1 s.

src/slic3r/GUI/LazyPage.hpp

@@ -57,6 +57,11 @@ public:
     // virtual from the book's ShowWithEffect() only for wxSHOW_EFFECT_NONE, the default.
     bool Show(bool show = true) override
     {
+        // Built before this page is shown, as an idle prebuild builds it. On MSW every control
+        // created or moved inside a shown window re-clips and repaints its shown siblings, so
+        // building a large panel into a shown page took seconds.
+        if (show && wxGetTopLevelParent(this)->IsShown())
+            this->ensure();
         const bool changed = wxPanel::Show(show);
         if (show) {
             // The book shows its first page as it is inserted, before startup has chosen the