Rework inlined union generation

Rework inlined union generation

Unions internal to a leaf union end up being incompletely generated,
as they lack stringValue(), 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:
- 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
- Class/InterfaceTemplate use different logic to emit the inner classes

JIRA: MDSAL-320
Change-Id: I626fb4ac42ae6528bc98b809bc33756e8daa08b9
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>