Add a pagenumber element subplugin

[abias]

For (really long) multipage certificates, it may be cumbersome to add page numbers by hand as text elements.

We propose to add a new pagenumber element subplugin, similar to the date element subplugin, which automatically shows the page number of the page.

Ideally, there will be multiple formats available, like

• 1
• Page: 1
• Page 1
• • 1 -

and ideally, the string "page" is translated in the plugin's language pack.