Plugin assumes that main content from the page is into #dokuwiki__content element

[eduardomozart]

The Bookcreator plugin fails to load the pages added to the book into "Create PDF book" page into Vector template because it assumes that the page content is into #dokuwiki__content, but Vector template uses #content. I create a pull request into Vector template to address this issue: foundata/dokuwiki-template-vector#66

But I believe that Bookcreator should use another method instead of relying into a preset HTML element name, or maybe offer the possibility to configure it's element name into plug-in options.

[Klap-in]

Unfortunately, the vector template lacks or has very delayed maintainance. Having a quick look, I think #dokuwiki__content is rather common for templates of DokuWiki. But I see it is used here for a very trivial thing, so I changed that. As bonus I improved the position of the dialog to be relative to the Add Namespace button.