Merge branch 'nicer-boxsizer-asserts'

Improve the error messages by indicating what should be done to avoid
the asserts and provide a way to disable them if really needed.

See https://github.com/wxWidgets/wxWidgets/pull/2375

interface/wx/sizer.h

@@ -1480,6 +1480,33 @@ public:
      */
     wxSizerFlags& CentreVertical();
 
+    /**
+        Globally disable checks for sizer flag consistency in debug builds.
+
+        By default, sizer classes such as wxBoxSizer and wxFlexGridSizer assert
+        when passed invalid flags, even though doing this usually doesn't
+        result in any catastrophic consequences and the invalid flags are
+        simply ignored later. Due to this, and the fact that these checks were
+        only added in wxWidgets 3.1, existing code may run into multiple
+        asserts warning about incorrect sizer flags use. Using this function
+        provides a temporary solution for avoiding such asserts when upgrading
+        to wxWidgets 3.1 from a previous version and will prevent such checks
+        from being done.
+
+        Please do note that correcting the code by removing the invalid flags
+        remains a much better solution as these asserts may be very helpful to
+        understand why some code using sizer flags doesn't work as expected, so
+        using this function, especially permanently, rather than a temporary
+        workaround, is @e not recommended.
+
+        Notice that the same effect as calling this function can be achieved by
+        setting the environment variable @c WXSUPPRESS_SIZER_FLAGS_CHECK to any
+        value.
+
+        @since 3.1.6
+     */
+    static void DisableConsistencyChecks();
+
     /**
         Sets the border in the given @a direction having twice the default
         border size.