feat: support directly setting dialog title text

Dialog does not currently support displaying dynamic strings
for titles, only static strings listed for translation. Accept
a new prop that explicitly states it is for setting the title
and have the web dialog prefer it over the titleKey.

react/features/base/dialog/components/AbstractDialog.js

         /**
          * Key to use for showing a title.
          */
-        titleKey: React.PropTypes.string
+        titleKey: React.PropTypes.string,
+
+        /**
+         * The string to use as a title instead of {@code titleKey}. If a truthy
+         * value is specified, it takes precedence over {@code titleKey} i.e.
+         * the latter is unused.
+         */
+        titleString: React.PropTypes.string
     }
 
     /**

react/features/base/dialog/components/Dialog.web.js

         return (
             <header>
                 <h2>
-                    { t(this.props.titleKey) }
+                    { this.props.titleString || t(this.props.titleKey) }
                 </h2>
             </header>
         );