Add keypath filtering to notifications

[papafe]

This PR adds support for keypath filtering in notifications

Description

Fixes ##1398

TODO

• Changelog entry
• Correct documentation
• Tests for lists/sets/dictionaries notifications
• Cleaning up internals
• Try to unify subscriptions path (keypath/full/shallow)

[papafe]

This works, and most of the tests are there and passing. It requires still some love though, so this is still a draft. I'll keep the TODO list updated in the PR description.

[nirinchev]

Haven't looked at the tests yet, but figured I'd add my comments so far and review the rest another time.

[papafe]

@nirinchev Fixed the final things and added the docs, this should be ready to merge.
I tried not to repeat myself in the docs, hopefully it makes sense overall

[coveralls-official]

Pull Request Test Coverage Report for Build 7832182965

• 84 of 90 (93.33%) changed or added relevant lines in 10 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.07%) to 80.888%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
Realm/Realm/DatabaseTypes/KeyPathCollection.cs	26	32	81.25%

Files with Coverage Reduction	New Missed Lines	%
Realm/Realm/Helpers/SerializationHelper.cs	1	53.53%

Totals
Change from base Build 7831586820:	0.07%
Covered Lines:	6663
Relevant Lines:	8093

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💛 - Coveralls