Add default ctors and Create() to wxDirDialog and wxFileDialog in wxOSX.

Allow two-step creation of these classes.

Closes #15316.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74476 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Add wxX11EventLoopSourcesManager stub implementation to fix wxX11 linking.

Asynchronous wxExecute() still doesn't work in wxX11 but at least the programs
using the library can be linked now.

See #15305.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74477 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775