library: Port Memory Monitor demo to Vala (#770)

* library: Port Memory Monitor demo to Vala

* Link `MemoryMonitorWarningLevel` in Memory Monitor Vala demo

Author: Diego-Ivan

src/Library/demos/Memory Monitor/main.vala

@@ -0,0 +1,37 @@
+#!/usr/bin/env -S vala workbench.vala --pkg libadwaita-1
+
+private HashTable<string, int?> cache;
+
+public void main () {
+  cache = new HashTable<string, int?> (string.hash, str_equal);
+  cache["a"] = 1;
+  cache["b"] = 2;
+  cache["c"] = 3;
+
+  var memory_monitor = MemoryMonitor.dup_default ();
+  memory_monitor.low_memory_warning.connect ((level) => {
+    /*
+     * Use inequalities for checking as new levels may be added in the future
+     * See https://valadoc.org/gio-2.0/GLib.MemoryMonitorWarningLevel.html
+     */
+    if (level >= MemoryMonitorWarningLevel.LOW) {
+      // Processes should free up unneeded resources
+      message ("Warning Level: Low");
+      drop_caches ();
+    }
+
+    if (level >= MemoryMonitorWarningLevel.MEDIUM) {
+      // Processes should try harder to free unneeded resources
+      message ("Warning Level: Medium");
+    }
+
+    if (level >= MemoryMonitorWarningLevel.CRITICAL) {
+      // The system will start terminating processes to reclaim memory
+      message ("Warning Level: Critical");
+    }
+  });
+}
+
+private void drop_caches () {
+  cache.remove_all ();
+}