Add support for data initialization function

[nirinchev]

Depends on #455 and #456, so will rebase on master once those go in.

This adds a Configuration property that allows developers to specify a function that imports an initial set of data the first time the Realm is opened.

final config = Configuration([Person.schema]);
config.initialDataCallback = (realm) {
  // realm is already in a write transaction
  realm.add(Person('peter'));
  realm.add(Person('george'));
};

Fixes #298

[coveralls]

Pull Request Test Coverage Report for Build 2115316376

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.841%

---

Totals
Change from base Build 2115293703:	0.0%
Covered Lines:	394
Relevant Lines:	429

---

💛 - Coveralls

[blagoev]

my PR review comments are all over the place. Sorry about that. Don't know how it happens

[nirinchev]

@blagoev @nielsenko @desistefanova this is now rebased on master and cleaned up, so should be ready for another look. It changed significantly since I integrated the GCHandle approach, so it's likely worth it reviewing the core functionality from scratch.