Add filter to control sorting in View entries query

[mrcasual]

This implements #1759.

To test, use this filter:

add_filter( 'gk/gravityview/view/entries/query/sorting-parameters', function ( $params ) {
	foreach ( $params as &$field ) {
		$field['is_numeric'] = true;
	}

	return $params;
} );

GravityMigrate data.

💾 Build file (81f6fa6).

[mrcasual]

@rafaehlers, please test.

[zackkatz]

I haven't tested this, but the code looks good.