implement manual update check settings item

mastodon/src/github/java/org/joinmastodon/android/updater/GithubSelfUpdaterImpl.java

@@ -26,6 +26,8 @@ import org.joinmastodon.android.api.MastodonAPIController;
 import org.joinmastodon.android.events.SelfUpdateStateChangedEvent;
 
 import java.io.File;
+import java.time.Instant;
+import java.time.LocalDateTime;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -101,6 +103,12 @@ public class GithubSelfUpdaterImpl extends GithubSelfUpdater{
 		}
 	}
 
+	@Override
+	public void checkForUpdates() {
+		setState(UpdateState.CHECKING);
+		MastodonAPIController.runInBackground(this::actuallyCheckForUpdates);
+	}
+
 	private void actuallyCheckForUpdates(){
 		Request req=new Request.Builder()
 				.url("https://api.github.com/repos/mastodon/mastodon-android/releases/latest")