REFACTOR: Move Page title / focus / counts logic to service

We had a handful of methods attached to the root `Discourse` object
related to focus and notification counts.

This patch pulls them out into a service called `document-title` for
updating the title, and a component called `d-document` to attach
and listen for browser events related to focus.

It also removes some computed properties and observers in favor of plain
old Javascript objects.

test/javascripts/services/document-title-test.js

@@ -0,0 +1,66 @@
+import { discourseModule } from "helpers/qunit-helpers";
+import { currentUser } from "helpers/qunit-helpers";
+
+discourseModule("service:document-title", {
+  beforeEach() {
+    this.documentTitle = this.container.lookup("service:document-title");
+    this.documentTitle.currentUser = null;
+    this.container.lookup("session:main").hasFocus = true;
+  },
+  afterEach() {
+    this.documentTitle.reset();
+  }
+});
+
+QUnit.test("it updates the document title", function(assert) {
+  this.documentTitle.setTitle("Test Title");
+  assert.equal(document.title, "Test Title", "title is correct");
+});
+
+QUnit.test(
+  "it doesn't display notification counts for anonymous users",
+  function(assert) {
+    this.documentTitle.setTitle("test notifications");
+    this.documentTitle.updateNotificationCount(5);
+    assert.equal(document.title, "test notifications");
+    this.documentTitle.setFocus(false);
+    this.documentTitle.updateNotificationCount(6);
+    assert.equal(document.title, "test notifications");
+  }
+);
+
+QUnit.test("it displays notification counts for logged in users", function(
+  assert
+) {
+  this.documentTitle.currentUser = currentUser();
+  this.documentTitle.currentUser.dynamic_favicon = false;
+  this.documentTitle.setTitle("test notifications");
+  this.documentTitle.updateNotificationCount(5);
+  assert.equal(document.title, "test notifications");
+  this.documentTitle.setFocus(false);
+  this.documentTitle.updateNotificationCount(6);
+  assert.equal(document.title, "(6) test notifications");
+  this.documentTitle.setFocus(true);
+  assert.equal(document.title, "test notifications");
+});
+
+QUnit.test(
+  "it doesn't increment background context counts when focused",
+  function(assert) {
+    this.documentTitle.setTitle("background context");
+    this.documentTitle.setFocus(true);
+    this.documentTitle.incrementBackgroundContextCount();
+    assert.equal(document.title, "background context");
+  }
+);
+
+QUnit.test("it increments background context counts when not focused", function(
+  assert
+) {
+  this.documentTitle.setTitle("background context");
+  this.documentTitle.setFocus(false);
+  this.documentTitle.incrementBackgroundContextCount();
+  assert.equal(document.title, "(1) background context");
+  this.documentTitle.setFocus(true);
+  assert.equal(document.title, "background context");
+});