Widgets - Google Translate: change a PHP 5.4 array syntax, check if title is defined

[eliorivero]

Fixes a couple of major issues spotted in https://github.com/Automattic/jetpack/pull/5386/files

Changes proposed in this Pull Request:

• change a PHP 5.4 array syntax into PHP 5.2
• check if title is defined to avoid issuing a PHP notice in Customizer
• add support for Customizer's Selective Refresh
• prefixes global JS function to avoid conflicts

Testing instructions:

• make sure you can see PHP errors, go to Customizer, add the Google Translate widget. There should not be a PHP notice

• when you update the widget in Customizer, make sure it's selectively refreshed, that is, only its portion is refreshed, rather than the whole page.

  @artpi I was concerned about these two changes, so I opened this. You can continue adding other changes here.

[artpi]

Thank you @eliorivero for these changes.
I added following modifications:

• add protection aginst direct access
• Change strip_tags to sanitize_text_field
• Use wp_enqueue_scripts to register frontend scripts

[zinigor]

Looks good, works good!