joinedmenu: add helper to join menus instead of nesting

In many cases, we have an "extra-menu" property that is used to allow
applications to join menus into the native menu for the widget. Previously,
this was done by nesting that menu in a section.

Doing so increases the complexity of the rules for GtkMenuTracker as you
may want different handling from inside of the section vs toplevel
sections.

If instead we synthetically glue the menus together, we have a much more
natural joining of menus as the application developer would expect for
their menu.

This also ports GtkLabel, GtkText, GtkPasswordEntry, and GtkTextView to
use the joined menu helper.

The joined menu helper comes originally from GNOME Builder and has had
extensive use there.

Fixes #4094

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