Pagination support

Hey there!
I try to add a custom admin menu/page with a subset of my pages. I want to filter all pages that have a special header variable set (`todo`) and list them. This way I can see any open TODOs for my page.

Now I've added a template similar to the comments plugin. Instead of using javascript I wanted to use the usual page collections and the pagination plugin. I've found no good way, only the [empty forum post](https://discourse.getgrav.org/t/pagination-within-admin-plugin/2671/2).

If you have any other idea how to realize such a TODO list, I am also happy to implement it differently.