Page layout doesnt get fully persisted

[MariusAlbrecht]

Describe the bug
Both the format setting (e.g. A5) and the units (px, mm, cm) sometimes get lost. I observe two scenarios two scenarios:

• not closing the app but only the document: only the "Format" value gets lost and is set to "Custom"
• closing the app: The "Format" value gets lost and is set to "Custom" and the units of "Width" and "Height" get lost and set to "Px"
  bot times the actual size stays the same (conversion takes place in scenario 2)

To Reproduce
Steps to reproduce the behavior:

1. Create document and assign it a Wayland (e.g. A5)
2. Save
3. Close (the document or the app)
4. Open (the app and then) the document again
5. Go to settings and see error(s)

Expected behavior
persist and restore all info. In particular, restore the predefined page layout choice

Console Output
n/a

Desktop (please complete the following information):

• OS: Ubuntu 23.10
• App Version: 0.10.2
• Installation Source: flatpak
• Desktop Environment: GNOME 45.2
• Display Server: Wayland

[flxzt]

The dimensions themselves are persistet, the A4/A5/.. choices are not included in the format.
Because the underlying combination of DPI and lengths can be unlimited for all formats, for example A4 at a lower DPI can be the same as A5 with higher DPI, it's difficult to restore them unambigiously.

But we could add storing the choice in the file format/user settings because I can see how it would be less confusing for users.