added wxICON_NONE and implement support for it in wxGTK (closes #2897)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61234 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/msgdlg.h

@@ -31,10 +31,16 @@
     @style{wxOK_DEFAULT}
         Makes the "OK" button default, this is the default behaviour and
         this flag exists solely for symmetry with @c wxCANCEL_DEFAULT.
+    @style{wxICON_NONE}
+        Displays no icon in the dialog if possible (an icon might still be
+        displayed if the current platform mandates its use). This style may be
+        used to prevent the dialog from using the default icon based on @c
+        wxYES_NO presence as explained in @c wxICON_QUESTION and @c
+        wxICON_INFORMATION documentation below.
     @style{wxICON_EXCLAMATION}
-        Displays an exclamation mark symbol.
+        Displays an exclamation, or warning, icon in the dialog.
     @style{wxICON_ERROR}
-        Displays an error symbol.
+        Displays an error icon in the dialog.
     @style{wxICON_HAND}
         Displays an error symbol, this is a MSW-inspired synonym for @c wxICON_ERROR.
     @style{wxICON_QUESTION}