Made deleting a page work.

Compared to before we are even removing the text from the page so the page
is probably deleted what also prevents adding new empty pages and frames to
the end. That's possible and easy now thanks to the great new architecture.

I also had to add getters for the QTextFrame::iterator's. That's probably
not optimal but then it makes more sense to do so to allow manipulation of
the content, like e.g. deleting a page, outside of the textlayout-lib.

There is one remaining problem; anchored objects like e.g. images are not
deleted as expected atm. That's still todo but then didn't worked before
too.