Split out structuring into its own phase

[LeonMatthesKDAB]

See #937

We want to avoid structuring while parsing, which leads to oddities like us having to parse all qobjects first, so we can add e.g. QEnums later.

So do this as its own phase before C++ generation, as we also probably don't need this for the Rust generation at all anyway.

What we need to associate:

• QEnums (see #942 )
• Signals
• Methods (e.g. invokables)
• Traits (e.g. threading, constructor, etc.)
• ...