Partial paginated search implementation

[ahmubashshir]

• engine: Add pagination support skeleton
• kitsu: Implement paginated search
• Gtk: Implement paginated search

Implementation

1. API libraries must implement paginated .search(), if pagination is unsupported, page count will be 1
2. If engine.search(...) is called with page=N ti returns paginated result, otherwise it returns an array of first n results (old behavior)
3. paginated results are returned like this [ results[], len(results[]), pagenum, pages ]

Related #687