Ensure that all opened editors' buffers are added to the project

Assigning a language mode happens when adding a buffer to the project,
so we need to guarantee this happens to all buffers used by visible editors.

src/workspace.js

@@ -497,6 +497,9 @@ module.exports = class Workspace extends Model {
           this.textEditorRegistry.maintainConfig(item),
           item.observeGrammar(this.handleGrammarUsed.bind(this))
         )
+        if (!this.project.findBufferForId(item.buffer.id)) {
+          this.project.addBuffer(item.buffer)
+        }
         item.onDidDestroy(() => { subscriptions.dispose() })
         this.emitter.emit('did-add-text-editor', {textEditor: item, pane, index})
       }