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

build/bakefiles/files.bkl

@@ -3028,6 +3028,7 @@ src/osx/iphone/window.mm
     src/gtk/calctrl.cpp
     src/gtk/dataview.cpp
     src/gtk/hyperlink.cpp
+    src/gtk/infobar.cpp
 </set>
 <set var="ADVANCED_GTK_NATIVE_HDR" hints="files">
     wx/gtk/animate.h
@@ -3035,6 +3036,7 @@ src/osx/iphone/window.mm
     wx/gtk/calctrl.h
     wx/gtk/dataview.h
     wx/gtk/hyperlink.h
+    wx/gtk/infobar.h
 </set>
 
 <set var="ADVANCED_GTK1_SRC" hints="files">