Add support for wxHELP button to wxMessageDialog.

Implement support for wxHELP for wxMSW, wxGTK and wxOSX/Cocoa (at least when
showing the message box from the main thread, there doesn't seem to be any way
to show more than three buttons with CFUserNotificationDisplayAlert() so
"Help" button is not supported when using it).

This is useful not only on its own, i.e. to allow the user to ask for help,
but also because it brings the total number of buttons supported by the
message dialog to 4, meaning that more choices can be offered to the user
(which is rarely, but not quite never, useful).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68537 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/msgdlg.h

@@ -22,6 +22,12 @@
         Puts Yes and No buttons in the message box. It is recommended to always
         use @c wxCANCEL with this style as otherwise the message box won't have
         a close button under wxMSW and the user will be forced to answer it.
+    @style{wxHELP}
+        Puts a Help button to the message box. This button can have special
+        appearance or be specially positioned if its label is not changed from
+        the default one. Notice that using this button is not supported when
+        showing a message box from non-main thread in wxOSX/Cocoa and it is not
+        supported in wxOSX/Carbon at all. @since 2.9.3.
     @style{wxNO_DEFAULT}
         Makes the "No" button default, can only be used with @c wxYES_NO.
     @style{wxCANCEL_DEFAULT}
@@ -116,6 +122,19 @@ public:
     */
     virtual void SetExtendedMessage(const wxString& extendedMessage);
 
+    /**
+        Sets the label for the Help button.
+
+        Please see the remarks in SetYesNoLabels() documentation.
+
+        Notice that changing the label of the help button resets its special
+        status (if any, this depends on the platform) and it will be treated
+        just like another button in this case.
+
+        @since 2.9.3
+     */
+    virtual bool SetHelpLabel(const ButtonLabel& help);
+
     /**
         Sets the message shown by the dialog.
 
@@ -190,7 +209,8 @@ public:
     virtual bool SetYesNoLabels(const ButtonLabel& yes, const ButtonLabel& no);
 
     /**
-        Shows the dialog, returning one of wxID_OK, wxID_CANCEL, wxID_YES, wxID_NO.
+        Shows the dialog, returning one of wxID_OK, wxID_CANCEL, wxID_YES,
+        wxID_NO or wxID_HELP.
 
         Notice that this method returns the identifier of the button which was
         clicked unlike wxMessageBox() function.
@@ -215,9 +235,9 @@ public:
     extended text and custom labels for the message box buttons, are not
     provided by this function but only by wxMessageDialog.
 
-    The return value is one of: @c wxYES, @c wxNO, @c wxCANCEL or @c wxOK
-    (notice that this return value is @b different from the return value of
-    wxMessageDialog::ShowModal()).
+    The return value is one of: @c wxYES, @c wxNO, @c wxCANCEL, @c wxOK or @c
+    wxHELP (notice that this return value is @b different from the return value
+    of wxMessageDialog::ShowModal()).
 
     For example:
     @code