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

docs/doxygen/overviews/envvars.h

@@ -32,5 +32,10 @@ wxWidgets programs.
          set it to @c "CURL" to force using libcurl-based implementation under
          MSW or macOS platforms where the native implementation would be chosen
          by default.}
+@itemdef{WXSUPPRESS_SIZER_FLAGS_CHECK,
+         If set, disables asserts about using invalid sizer flags in the code.
+         This can be helpful when running older programs recompiled with
+         wxWidgets 3.1 or later, as these asserts are mostly harmless and can
+         be safely ignored if the code works as expected.}
 */