Image captions from context menu

Based on the recent patch for context menu, allows the user to view title tag content for images (useful for webcomics and such).  

The method isn't perfect; there isn't an easy way to get a direct reference to the tag, so we scan the article source for an image with the same src as the clicked one, then pull the title tag from there. This will fail if the same image is used multiple times in an article with a different title, but is ok for most usage (though I understand if you reject the patch for imperfection...)