Add a 'gp_before_translation_table' hook

[amieiro]

Problem

We don't have a hook to update the content before the translation table, as we have after it, with the gp_after_translation_table hook, added in #1665.

This hook is needed here.

Solution

This PR adds an action to add content before the translation table is displayed.

Testing Instructions

To test this PR, you can use a code like this:

add_action( 'gp_before_translation_table',
	function () {
		echo '<div>This text should be before the table</div>';
	} );

Screenshots or screencast