feat(dialog): persist global sort option

[avnyu]

Fixes: pop-os/xdg-desktop-portal-cosmic#258
Fixes: #1963

Allows file dialogs to save and load sort options via State.dialog_sort_name. Previously, dialogs always defaulted to descending modified time.

Tested using the dialog example. Changing sort options updates $HOME/.local/state/cosmic/com.system76.CosmicFiles/v1/dialog_sort_name, and opening new dialogs applies the stored option.
Also tested in archlinux using xdg-cosmic-portal-cosmic package built from archlinux PKGBUILD + patch to change cosmic-files dependency to this branch.

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