Add default values for wxRichMessageDialog ctor arguments.

Both "caption" and "style" arguments were documented as having default values,
but didn't really have them. Fix this by actually adding them.

Closes #16524.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77535 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775