Java: Make `Annotatable` abstract / refine `Annotation.getAnnotatedElement()`

Currently `Annotatable` is a regular CodeQL class which extends `Element`. This makes it cover a lot of entity types which are not actually annotatable, such as all expressions, statements (only some statements are annotatable) and files. This is probably not a big issue, but it prevents for example the CodeQL compiler from detecting incompatible casts.

A related issue is how `Annotation.getAnnotatedElement()` is defined (and that it currently has `Element` as result type). At the moment it erroneously has `Annotation` and `ArrayInit` as result when they contain nested annotations, e.g. `@Outer(@Innter)`, or `@Outer({@Inner(1), @Inner(2)})`.
Example query:
```codeql
import java

from Annotatable a
where not (
a instanceof FieldDeclaration
or a instanceof Member
or a instanceof Package
or a instanceof RefType
or a instanceof TypeAccess
or a instanceof Variable
)
select a, a.getPrimaryQlClasses(), a.getAnAnnotation()
```
[Query Console link](https://lgtm.com/query/6189668047736089840/)

If you give me a hint what the desired solution to this is, I can try to give it a go, unless you want to fix this yourselves of course.

Start with the Java library definitions of Annotatable and Annotation.getAnnotatedElement(), then run the example query from the issue to inspect the current results. Determine the intended set of annotatable element types and the correct result type for nested annotations; done means incompatible casts can be detected and nested annotations no longer produce Annotation or ArrayInit as annotated elements.

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