Add margin to the right of the message in wxGenericProgressDialog.

For some reason the border was used only on the left side but not on the right
one, resulting in the ugly looking label flush to the right dialog border if
it was long enough.

Closes #16094.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76156 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775