Add wxEVT_WEBVIEW_CHROMIUM_MESSAGE_RECEIVED

This allows to handle IPC messages from custom JS functions in the code
using wxWebViewChromium.

interface/wx/webview_chromium.h

@@ -254,6 +254,17 @@
 
     - SetRoot() allows to use the given directory for all application files.
 
+
+    @section events Events
+
+    wxWebViewChromiumEvent is a specific event generated when a message from
+    the renderer process (in which any user-defined JavaScript functions are
+    executing) is received in the browser process (containing UI). Such
+    messages are sent using `CefFrame::SendProcessMessage(PID_BROWSER)` from
+    the renderer process, but describing Chromium IPC in details is out of
+    scope of this document, please see Chromium documentation for more
+    information.
+
     @since 3.3.0
     @library{wxwebview}
     @category{webview}
@@ -292,3 +303,63 @@ public:
      */
     void SetRoot(const wxFileName& rootDir);
 };
+
+
+/**
+    Event class for events generated exclusively by wxWebViewChromium.
+
+    See wxWebViewEvent for other events also generated by wxWebViewEvent.
+
+    Currently there is a single event type corresponding to this event:
+    `wxEVT_WEBVIEW_CHROMIUM_MESSAGE_RECEIVED`. To use it (this example assumes
+    that the required CEF headers are included):
+
+    @code
+    webview->Bind(
+        wxEVT_WEBVIEW_CHROMIUM_MESSAGE_RECEIVED,
+        [this](wxWebViewChromiumEvent& event) {
+            if ( event.GetMessageName() == "MyCustomRequest" ) {
+                CefRefPtr<CefListValue> arguments = event.GetMessage().GetArgumentList();
+                CefRefPtr<CefProcessMessage> response = CefProcessMessage::Create("MyCustomResponse");
+
+                // ... do whatever is needed to construct the response ...
+
+                event.GetFrame().SendProcessMessage(PID_RENDERER, response);
+            } else {
+                // Not our message.
+                event.Skip();
+            }
+        }
+    );
+    @endcode
+
+    @since 3.3.0
+    @library{wxwebview}
+    @category{webview}
+**/
+class wxWebViewChromiumEvent : public wxCommandEvent
+{
+public:
+    /**
+        Get the associated browser frame.
+
+        This object can be used to call `SendProcessMessage(PID_RENDERER)` on
+        it to send the reply back to the renderer process.
+    */
+    CefFrame& GetFrame() const;
+
+    /**
+        Get the actual message.
+
+        Message arguments can be used in the event handler to get information
+        associated with this message.
+    */
+    CefProcessMessage& GetMessage() const;
+
+    /**
+        Get the message name.
+
+        This is just a convenient wrapper for `GetMessage().GetName()`.
+    */
+    wxString GetMessageName() const;
+};