Fix wxWebView::RunScript() with string containing backslashes

Escape backslashes in wxJSScriptWrapper to allow strings with
backslashes in them to work again -- this was broken while implementing
support for returning values from JavaScript to C++.

See https://github.com/wxWidgets/wxWidgets/pull/741

include/wx/private/jsscriptwrapper.h

@@ -38,7 +38,7 @@ public:
 
         // Adds one escape level if there is a single quote, double quotes or
         // escape characters
-        wxRegEx escapeDoubleQuotes("(\\\\*)(['\"\n\r\v\t\b\f])");
+        wxRegEx escapeDoubleQuotes("(\\\\*)([\\'\"\n\r\v\t\b\f])");
         escapeDoubleQuotes.Replace(&m_escapedCode,"\\1\\1\\\\\\2");
     }