Create Refresher class (and IRefreshable interface)

client/src/meson.build

@@ -19,6 +19,7 @@ lib = library(
     'der.vala',
     'main_window.vala',
     'periodic.vala',
+    'refresher.vala',
     'request.vala',
     'response.vala',
     'session_manager.vala',

client/src/refresher.vala

@@ -0,0 +1,23 @@
+namespace StudySystemClient {
+	public interface IRefreshable {
+		public abstract async void refresh();
+	}
+
+	private class Refresher : Periodic {
+		private weak IRefreshable target;
+
+		public Refresher(IRefreshable target, uint period_ms) {
+			base(period_ms);
+			this.target = target;
+		}
+
+		protected override void task() {
+			if (target != null) {
+				Idle.add(() => {
+						target.refresh.begin();
+						return false;
+					});
+			}
+		}
+	}
+}