Add access to user agent for wxWebView

Allow setting a custom user agent for a webview.
Also allow access to the current user agent.

interface/wx/webview.h

@@ -889,6 +889,28 @@ public:
     */
     virtual bool IsAccessToDevToolsEnabled() const;
 
+    /**
+        Specify a custom user agent string for the web view.
+        Returns @true the user agent could be set.
+
+        If your first request should already use the custom user agent
+        please use two step creation and call SetUserAgent() before Create().
+
+        @note This is not implemented for IE. For Edge SetUserAgent()
+            MUST be called before Create().
+
+        @since 3.1.5
+    */
+    virtual bool SetUserAgent(const wxString& userAgent);
+
+    /**
+        Returns the current user agent string for the web view.
+
+        @since 3.1.5
+    */
+    virtual wxString GetUserAgent() const;
+
+
     /**
         @name History
     */