AgendaEventRow: create get_timespan_label

[danirabbit]

Moves timespan label to its own function

• Eliminates a bunch of nesting
• Will be easier to replace name_label and datetime_label with Granite.HeaderLabel in the future

Also:

• Replace a string comparison with a DateTime.equal
• Don't use no_show_all etc for hiding/showing which don't exist in Gtk4. Instead simply attach/remove
• No spaces around en dash for ranges per style guide