Draft: shell: fixes related to opening documents

This removes the metadata, and uses a gsettings key

@sophie-h @FineFindus @jfft would you mind testing this? I think this should fix most of the issues opening documents on weird sizes.

Fixes #134 (closed)

Fixes #132 (closed)

Fixes #20 (closed)

TODOs:

• Handle maximization state (#106 (closed))
  When closing in maximized state, it needs to save the maximization property independently, without affecting the previously saved width+height
• Fix the crasher when opening a PDF via the Nightly flatpak version of Nautilus (see comment below)
• More testing
• Rename functions, to make clear what's before and what after
• Bind settings as per https://developer.gnome.org/documentation/tutorials/save-state.html

Depends !204 (merged) just because that simplifies the opening paths that allow reasoning about this.