datetime-page: Minor tweaks in the "Date & Time" dialog

Use AdwPreferencesPage/Group

This simplifies the .ui code a bit and keep consistency with other dialogs.

Use AdwActionRow in the Month row

The Month row is a CcListRow which displays the selected month using the secondary-label property from CcListRow. However, the month appears dimmed (because secondary-label uses the .dim-label style class) and the Month row is supposed to mimick AdwComboRow, which doesn't dim the selected item. To fix this, use an AdwActionRow instead.

Set a content width in the "Date & Time" dialog

The GtkListBox from the "Date & Time" dialog is currently what gives the dialog width, but this is an unusual approach that other dialogs from Settings don't use.

To fix this, use the content-width property from AdwDialog. The dialog width is now almost the same as before this change.

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