Use new wxAppTraits::SafeMessageBox() in wxSafeShowMessage()

This allows to show message boxes in ports other than wxMSW too by doing
it only when it is safe, i.e. when the GUI is initialized, while still
keeping the old code directly using the native MessageBox() function for
MSW for maximal robustness.

interface/wx/apptrait.h

@@ -133,5 +133,28 @@ public:
         Returns @true to suppress subsequent asserts, @false to continue as before.
     */
     virtual bool ShowAssertDialog(const wxString& msg) = 0;
+
+    /**
+        Shows a message box with the given text and title if possible.
+
+        In some ports, e.g. wxMSW, a message box will always be shown, while in
+        the other ones it will be only done if the GUI is available (e.g. X11
+        display was successfully opened for X11-based ports) and the function
+        simply returns @false without doing anything otherwise.
+
+        This function is safe in the sense that it can always be called, even
+        before creating wxApp, similarly to wxSafeShowMessage() which is
+        implemented by calling this function and then logging the message to
+        standard error stream if it returned @false.
+
+        @since 3.1.5
+
+        @param text
+            The text to show to the user.
+        @param title
+            The title of the message box shown to the user.
+        @return @true if the message box was shown or @false otherwise.
+     */
+    virtual bool SafeMessageBox(const wxString& text, const wxString& title) = 0;
 };