Enable setMultiple method under basefield

[ishannz]

Enable setMultiple method under basefield.

Usage:

$fields->addFieldToTab('Root.Main', ImageField::create('ImageGallery')->setMultiple(true));
$fields->addFieldToTab('Root.Main', MediaField::create('VideoGallery')->setMultiple(true));
$fields->addFieldToTab('Root.Main', FileField::create('Downloads')->setMultiple(true));

Based on the issue I raised:
#185