Allow to leave another window enabled in wxWindowDisabler

Allow specifying another window to "skip", i.e. not to disable, in
wxWindowDisabler.

This is not used yet, but will be in the upcoming commit.

include/wx/utils.h

@@ -732,8 +732,8 @@ public:
     // it doesn't do anything
     explicit wxWindowDisabler(bool disable = true);
 
-    // ctor disables all windows except winToSkip
-    explicit wxWindowDisabler(wxWindow *winToSkip);
+    // ctor disables all windows except the given one(s)
+    explicit wxWindowDisabler(wxWindow *winToSkip, wxWindow *winToSkip2 = NULL);
 
     // dtor enables back all windows disabled by the ctor
     ~wxWindowDisabler();