Link dialog hook

```
Hello, here is patch that adds hook to display custom link dialog, just like
the image dialog.

I needed it when integrating Pagedown into CMS, where I want to link another
components in the application. Now I can show dialog which allows to pick such
component.
```

Original issue reported on code.google.com by `jkuf...@gmail.com` on 1 Mar 2015 at 10:09

Attachments:
- [pagedown.patch](https://storage.googleapis.com/google-code-attachments/pagedown/issue-84/comment-0/pagedown.patch)

修補程式檔案 pagedown.patch 顯示了新增自訂連結對話方塊 hook 的變更，其方式類似於影像對話方塊。首先檢查程式碼庫中現有的影像對話方塊實作，以了解 hook 模式。接著套用修補程式查看修改內容，再透過在示範中整合自訂連結對話方塊進行測試，以驗證功能。

由索引模型根據 Issue 內容生成。