Dashboard Text Panel: improve feed when changing Description

Summary:
When creating a Dashboard Text Panel, from example from here:

http://phorge.localhost/dashboard/panel/edit/?panelType=text

After you change its Description,

Before this change, this was the feed (but you must have Developer Mode):

    foo.bar edited this object (transaction type "text.text").

After this change, this is the feed:

    foo.bar edited the the panel text. (Show Details)

So, not only we show a nice title, but we also allow to see details.

| Before     | After      |
|------------|------------|
| {F2182938} | {F2182943} |

Credits:

- LegalpadDocumentTextTransaction for the getTitle() method
- ManiphestTaskDescriptionTransaction for the details

Closes T15817

Test Plan:
- Create a Text Panel from /dashboard/panel/edit/?panelType=text
- Change description
- See the new feed
- Click on "Show Details". Also that works. You see A/B changes.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15817

Differential Revision: https://we.phorge.it/D25625

src/applications/dashboard/xaction/panel/PhabricatorDashboardTextPanelTextTransaction.php

@@ -9,6 +9,25 @@ final class PhabricatorDashboardTextPanelTextTransaction
     return 'text';
   }
 
+  public function getTitle() {
+    return pht(
+      '%s updated the panel text.',
+      $this->renderAuthor());
+  }
+
+  public function hasChangeDetailView() {
+    return true;
+  }
+
+  public function newChangeDetailView() {
+    $viewer = $this->getViewer();
+
+    return id(new PhabricatorApplicationTransactionTextDiffDetailView())
+      ->setViewer($viewer)
+      ->setOldText($this->getOldValue())
+      ->setNewText($this->getNewValue());
+  }
+
   public function newRemarkupChanges() {
     $changes = array();