Ensure custom title bar is always updated when document.title changes

This commit uses a new private API on Workspace that emits an event
every time the window title gets updated (e.g. as a result of an active
pane item changing, the project paths changing, etc.).

This fixes a bug that left the custom title bar with a stale
document.title under some circumstances.

Signed-off-by: Jason Rudolph <jasonrudolph@github.com>

src/title-bar.coffee

@@ -10,7 +10,7 @@ class TitleBar
 
     @element.addEventListener 'dblclick', @dblclickHandler
 
-    @workspace.onDidChangeActivePaneItem => @updateTitle()
+    @workspace.onDidChangeWindowTitle => @updateTitle()
     @themes.onDidChangeActiveThemes => @updateWindowSheetOffset()
 
     @updateTitle()

src/workspace.js

@@ -587,6 +587,7 @@ module.exports = class Workspace extends Model {
 
     document.title = titleParts.join(' \u2014 ')
     this.applicationDelegate.setRepresentedFilename(representedPath)
+    this.emitter.emit('did-change-window-title')
   }
 
   // On macOS, fades the application window's proxy icon when the current file
@@ -864,6 +865,10 @@ module.exports = class Workspace extends Model {
     return this.emitter.on('did-add-text-editor', callback)
   }
 
+  onDidChangeWindowTitle (callback) {
+    return this.emitter.on('did-change-window-title', callback)
+  }
+
   /*
   Section: Opening
   */