Fix zoom center

[tmoerschell]

Fixes #5269

This is depending on two other things:

• Merging commits from Rebased: Fix zoom jumping and zoom steps (Partial Zoom Refactor) #5108 from release-1.2 to master (there is just one assert() that has to be changed to xoj_assert() for this)
• We have to clear up what we want to do in PR Create constant padding page layout option #5270. If there are changes to that, this PR will have to be reworked as well.

I have included the commits from the above points for now, so you can see / test the changes. I will rebase/do whatever, when we decide to move forward and have an appropriate base to merge upon. As such, the only really relevant commit here is the last one (017450c).

Question to maintainers: when should I open a PR for merging the commits of #5108 onto master? Now, when the other issue is cleared up, or not at all (merging them through this PR)?

[bhennion]

Question to maintainers: when should I open a PR for merging the commits of #5108 onto master? Now, when the other issue is cleared up, or not at all (merging them through this PR)?

I'll do it this weekend if you want.

[tmoerschell]

Rebased onto master now. Thanks @bhennion !