CalendarView: GTK4 prep

[danirabbit]

GTK4 prep and code style:

• use SPDX header
• Reduce variable scope and organize
• rename glib settings show_weeks as settings
• Menubutton use menu model directly instead of building a gtk.menu manually
• Brackets with if
• Explicit vertical and horizontal expands
• Explicitly scope functions
• get rid of single-use function to build the grid
• Use a revealer to show/hide weeks header spacer and bind to show_weeks setting