PDF export to cater for better heading formatting

To demonstrate this limitation, please refer to the following steps:

1. Create a space
2. In the "Home" page, add the following content:

   h1. Heading A
   h2. Heading B
   h1. Heading C
   

3. Create a child page "Child" with this content:

   h1. Heading D
   h2. Heading E
   h1. Heading F
   

4. Export the space to PDF

In the resultant PDF:

• "Home" is displayed as the prime heading (h1)
• "Heading A" is displayed as h2, "Heading B" is displayed as h3, and so on
• However, "Child" (the child page title) is displayed as h2 (not h1), "Heading D" is displayed as h3 and so on <-- this is the limitation

In other words, you can see that on the first page the headings are increased by one:

From the second (child page) on, headings are increased by two:

Because of this reason it's not possible to do a senseful page numbering with css counters. If the headings would be shifted on all pages in the same way, this would be no problem.

A suggestion would be that Page Titles always get h1 in the PDF export and the content headings are always only shifted by 1. Additionally (to keep the possibility to define something like a title page with the very first page title), the very first page title could get something like <h1 class="very_first_page_title">.