Add vala code sample for Breakpoints demo (#511)

Author: SpikedPaladin

src/Library/demos/Breakpoints/main.vala

@@ -0,0 +1,8 @@
+#! /usr/bin/env -S vala workbench.vala --pkg libadwaita-1
+
+public void main () {
+  var breakpoint = (Adw.Breakpoint) workbench.builder.get_object ("breakpoint");
+
+  breakpoint.apply.connect (() => message ("Breakpoint Applied"));
+  breakpoint.unapply.connect (() => message ("Breakpoint Unapplied"));
+}