Rename wxEllipsizeFlags elements to avoid confusion with wxEllipsizeMode.

We shouldn't use the same "wxELLIPSIZE_" prefix for two different enums, so
use wxELLIPSIZE_FLAGS one for wxEllipsizeFlags (they should be used less often
than wxEllipsizeMode so it's better to keep the short prefix for the latter).

Also add wxELLIPSIZE_FLAGS_NONE flag.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62432 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775