Convert margin spinedits

I tried to handle the issues of this ticket: #4948 (comment 2532345342)

Summary: I replaced the SpinEdits by regular Gtk::Entries and made sure, that the output of those are formatted according to the margin field in the toolbar (and not in the popup). With the new inputs, you can enter any unit (not only the document unit) with as many digits as you like.

I have one topic where I would be happy to have some feedback: When you open the popup for the very first time after inkscape has started, the method getSelectedPage of the page-manager returns null, which leads to the ugly behaviour, that the inputs remain empty then. It was like this before and I'm missing some deeper knowledge where this might come from. Maybe one of the reviewers can give me a hint or we keep it for now.

Fixes #4948 (closed)