Library entries for expressions

[sonnyp]

GTK Expressions are pretty powerful and easy to use with Blueprint but the documentation is awful, specially for newcomers.
We should have a Library entry for them.

• https://docs.gtk.org/gtk4/class.Expression.html
• https://jwestman.pages.gitlab.gnome.org/blueprint-compiler/reference/expressions.html
• Define Gtk closure expressions in code when the UI file is NOT a template. Workbench#667