Added missing DebuggerDisplay

[papafe]

This PR adds DebuggerDisplay to some Realm types. In particular debugger display contains:

• RealmObjectBase: IsValid
• RealmValueType: Type - ToString() --> This can be useful to differentiate between different numerical types with similar string representation, for instance

Regarding the rest of the Realm types:

• Collection types (Set, Dictionary, List, Result) and their superclass RealmCollectionBase don't have anything meaningful to display in my opinion. For the "proper" collection types (Set, Dictionary, List), the parameter Count was already in the DebuggerDisplay attribute, and that seems fine.
• RealmInteger already prints the containing value in ToString(), and that also seems fine enough.

Fixes #2047

[nirinchev]

I think we can be a bit more precise with these. Currently, this is what I see when I hover over a RealmObject - it's not particularly useful:

I think we can make it nicer by including the type of the object - perhaps by introducing a private property and using that for the debugger display? I was thinking of something like Person { Id = 123 }, IsValid = true. We can get the primary key property name from ObjectSchema.PrimaryKeyProperty and the primary key value from _metadata.Helper.TryGetPrimaryKeyValue(this).

For RealmValue, we're currently outputting the namespaced type, which is long and boring. I was thinking we could replace it with something like RealmValue, Type = String, Value = "abc" and for objects, just RealmValue, Person { Id = 123 }, IsValid = true.

[papafe]

I think it makes sense to add the primary key to the DebugDisplay for RealmObjectBase. I am not sure about adding the Type though, I think it would not be a useful information, as the majority of the time the developer would know already that. Besides, it's possible to see it in the "Locals" debug window.

Regarding RealmValue, I agree on adding Type = String, Value = "abc", but I don't think that adding the string RealmValue in front is useful, for the same reason as before. In this case, though, it definitely makes sense to print the object type.

[papafe]

I have added some improvements, we can probably discuss about them :)