GuestsPanel: GObject style, cleanup

[danirabbit]

• Use SPDX header
• Subclass Box instead of Grid
• Take ICal component as construction arg instead of parent dialog. We only need the component here since EventDialog's can_edit is never set
• Don't cache style context, makes an easier replace when we port to GTK4
• Set properties when instantiating
• Explicit h and v expand
• Set mnemonic widget for "Invitees" header so the list has a label for the screen reader
• Use child property where appropriate
• minor code style changes