Rework inlined union generation

Rework inlined union generation

Unions internal to a leaf union end up being incompletely generated,
as they lack getValue(), hashCode(), equals() and do not correctly
bind to its enclosing builder -- leading to a generated code not being
compilable.

There are multiple issues here, all of which are addressed in this patch:
- value/hashCode/equals properties are created in the wrong place
- property return type is set to the builder, not its product
- union builder type is not set as a union
- builders for nested types are not correctly emitted

JIRA: MDSAL-320
Change-Id: I60697c74669d4508922ea3ee22e2f0fe5d99b322
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>