Added native wxInfoBar implementation for wxGTK.

Added native wxInfoBar implementation for wxGTK.

Straightforward implementation of wxInfoBar using GtkInfoBar widget available
in GTK+ 2.18.

Some side effects of this change:

- Rename wxInfoBar version in wx/generic/infobar.h to wxInfoBarGeneric and
  define wxInfoBar in wx/infobar.h.
- Also change default value of flags argument to ShowMessage() to
  wxICON_INFORMATION from wxICON_NONE as the default colour for wxICON_NONE
  messages is surprisingly ugly in GTK native version.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62276 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775