Fix SwiftypeEngineSlug field in the CMS

Change text field name from SwiftypeEngineSlug/ID to SwiftypeEngineSlug to enable proper saving

Fix dependency issue in SwiftypeSiteConfig

Add dependency on SilverStripe\Forms\LiteralField in SwiftypeSiteConfig to fix issue in the CMS admin section.

First release working with SS4

First beta release of the Swiftype module contains

• Supports Swiftype crawler for indexing
• Metadata tag injected by SwiftypeMetaTag classes by a Content extension
• Admin section to configure Swiftype codes
• Swiftype code template
• Trigger re-indexing via API by SiteTree Extension.